Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

[I'm combining the messages again, since I feel a bit bad, when I write
so many mails to the list ;) ]
But from my side, feel free to split up as much as you want (perhaps
not single characters or so ;) )


On Thu, 2015-12-17 at 04:06 +, Duncan wrote:
> Just to mention here, that I said "integrity management features",
> which 
> includes more than checksumming.  As Austin Hemmelgarn has been
> pointing 
> out, DBs and some VMs do COW, some DBs do checksumming or at least
> have 
> that option, and both VMs and DBs generally do at least some level
> of 
> consistency checking as they load.  Those are all "integrity
> management 
> features" at some level.
Okay... well, but the point of that whole thread was obviously data
integrity protection in the sense of what data checksumming does in
btrfs for CoWed data and for meta-data.
In other words: checksums at some blockleve, which are verified upon
every read.



> As for bittorrent, I /think/ the checksums are in the torrent files 
> themselves (and if I'm not mistaken, much as git, the chunks within
> the 
> file are actually IDed by checksum, not specific position, so as long
> as 
> the torrent is active, uploading or downloading, these will by
> definition 
> be retained).  As long as those are retained, the checksums should
> be 
> retained.  And ideally, people will continue to torrent the files
> long 
> after they've finished downloading them, in which case they'll still
> need 
> the torrent files themselves, along with the checksums info.
Well I guess we don't need to hook up ourselves so much on the p2p
formats.
They're just one examples, even if these would actually be integrity
protected in the sense as described above, well, fine, but there are
other major use cases left, for which this is not the case.

Of course one can also always argue, that users can then manually move
the files out of the no-CoWed area or manually create their own
checksums as I do and store them in XATTRS.
But all this is not real proper full checksum protection: there are
gaps, where things are not protected and normal users may simply not
do/know all this (and why shouldn't they still benefit from proper
checksumming if we can make it for them).
IMHO, even the argument that one could manually make checksums or move
the file to CoWed area, while the e.g. downloaded files are still in
cache doesn't count: that wouldn't work for VMs, DBs, and certainly not
for torrent files larger than the memory.


> Meanwhile, if they do it correctly there's no window without
> protection, 
> as the torrent file can be used to double-verify the file once moved,
> as 
> well, before deleting it.
Again, would work only for torrent-like files, not for VM images, only
partially for DBs... plus... why requiring users to make it manually,
if the fs could take care of it.







On Thu, 2015-12-17 at 05:07 +, Duncan wrote:
> > In kinda curios, what free space fragmentation actually means here.
> > 
> > Ist simply like this:
> > +--+-+---++
> > > F|  D  | F |D   |
> > +--+-+---++
> > Where D is data (i.e. files/metadata) and F is free space.
> > In other words, (F)ree space itself is not further subdivided and
> > only
> > fragmented by the (D)ata extents in between.
> > 
> > Or is it more complex like this:
> > +-++-+---++
> > >  F  |  F |  D  | F |D   |
> > +-++-+---++
> > Where the (F)ree space itself is subdivided into "extents" (not
> > necessarily of the same size), and btrfs couldn't use e.g. the
> > first two
> > F's as one contiguous amount of free space for a larger (D)ata
> > extent
> At the one level, I had the simpler f/d/f/d scheme in mind, but that 
> would be the case inside a single data chunk.  At the higher file
> level, 
> with files significant fractions of the size of a single data chunk
> to 
> much larger than a single data chunk, the more complex and second
> f/f/d/f/d case would apply, with the chunk boundary as the
> separation 
> between the f/f.
Okay, but that's only when there are data chunks that neighbour each
other... since the data chunks are rather big normally (1GB) that
shouldn't be such a big issue,... so I guess the real world looks like
this:
 DC#1                  DC#2
...+-...
...---+|+--+-+-
--++
... F |||     F|  D  | F |D   |
...---+|+
--+-+---++
...++-...
(with DC = data chunk)

but it could NOT look like this:
 DC#1                  DC#2
...+-...
...---+|+-++-+---++
... F |||  F  |  F |  D  | F
|D   |
...---+|+-++-+---++
...++-
...
in other words, there could be =2 adjacent free
space "extents", when these are actually parts of different
neighbouring chunks, but there could NOT be >=2 adjacent free space
"extents" 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

On Wed, 2015-12-09 at 16:36 +, Duncan wrote:
> But... as I've pointed out in other replies, in many cases including
> this 
> specific one (bittorrent), applications have already had to develop
> their 
> own integrity management features
Well let's move discussion upon that into the "dear developers, can we
have notdatacow + checksumming, plz?" where I showed in one of the more
recent threads that bittorrent seems rather to be the only thing which
does use that per default... while on the VM image front, nothing seems
to support it, and on the DB front, some support it, but don't use it
per default.


> In the bittorrent case specifically, torrent chunks are already 
> checksummed, and if they don't verify upon download, the chunk is
> thrown 
> away and redownloaded.
I'm not a bittorrent expert, because I don't use it, but that sounds to
be more like the edonkey model, where - while there are checksums -
these are only used until the download completes. Then you have the
complete file, any checksum info thrown away, and the file again being
"at risk" (i.e. not checksum protected).


> And after the download is complete and the file isn't being
> constantly 
> rewritten, it's perfectly fine to copy it elsewhere, into a dir where
> nocow doesn't apply.
Sure, but again, nothing the user may automatically do, and there's
still the gap between the final verification from the bt software, to
the time it's copied over.
Arguably, that may be very short, but I see no reasons to make any
breaks in the everything-verified chain from the btrfs side.


>   With the copy, btrfs will create checksums, and if 
> you're paranoid you can hashcheck the original nocow copy against the
> new 
> checksummed/cow copy, and after that, any on-media changes will be
> caught 
> by the normal checksum verification mechanisms.
As before... of course you're right that one can do this, but nothing
that happens per default.
And I think that's just one of the nice things btrfs would/should give
us. That the filesystem assures that data is valid, at least in terms
of storage device and bus errors (it cannot protect of course against
memory errors or that like).


> > Hmm doesn't seem really good to me if systemd would do that, cause
> > it
> > then excludes any such files from being snapshot.
> 
> Of course if the directories are already present due to systemd
> upgrading 
> from non-btrfs-aware versions, they'll remain as normal dirs, not 
> subvolumes.  This is the case here.
Well, even if not, because one starts from a fresh system... people may
not want that.


> And of course you can switch them around to dirs if you like, and/or 
> override the shipped tmpfiles.d config with your own.
... sure but, people may not even notice that.
I don't think such a decision is up to systemd.
Anyway, since we're btrfs here, not systemd, that shouldn't bother us
;)


> > > and small ones such as the sqlite files generated by firefox and
> > > various email clients are handled quite well by autodefrag, with
> > > that
> > > general desktop usage being its primary target.
> > Which is however not yet the default...
> Distro integration bug! =:^)
Nah,... really not...
I'm quite sure that most distros will generally decide against
diverting from upstream in such choices.



> > It feels a bit, if there should be some tools provided by btrfs,
> > which
> > tell the users which files are likely problematic and should be
> > nodatacow'ed
> And there very well might be such a tool... five or ten years down
> the 
> road when btrfs is much more mature and generally stabilized, well
> beyond 
> the "still maturing and stabilizing" status of the moment.
Hmm let's hope btrfs isn't finished only when the next-gen default fs
arrives ;^)



> But it can be the case that as filesystem fragmentation levels rise,
> free-
> space itself is fragmented, to the point where files that would
> otherwise 
> not be fragmented as they're created once and never touched again,
> end up 
> fragmented, because there's simply no free-space extents big enough
> to 
> create them in unfragmented, so a bunch of smaller free-space extents
> must be used where one larger one would have been used had it
> existed.
In kinda curios, what free space fragmentation actually means here.

Ist simply like this:
+--+-+---++
|     F    |  D  | F |    D   |
+--+-+---++
Where D is data (i.e. files/metadata) and F is free space.
In other words, (F)ree space itself is not further subdivided and only
fragmented by the (D)ata extents in between.

Or is it more complex like this:
+-++-+---++
|  F  |  F |  D  | F |    D   |
+-+
+-+---++
Where the (F)ree space itself is subdivided
into "extents" (not necessarily of the same size), and btrfs couldn't
use e.g. the first two F's as one contiguous amount of free space for a
larger (D)ata extent of that size:
+--+-+---++
|    
D    |  D  | F |    D   |

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

On Mon, 2015-12-14 at 10:51 +, Duncan wrote:
> > AFAIU, the one the get's fragmented then is the snapshot, right,
> > and the
> > "original" will stay in place where it was? (Which is of course
> > good,
> > because one probably marked it nodatacow, to avoid that
> > fragmentation
> > problem on internal writes).
> 
> No.  Or more precisely, keep in mind that from btrfs' perspective, in
> terms of reflinks, once made, there's no "original" in terms of
> special 
> treatment, all references to the extent are treated the same.
Sure... you misunderstood me I guess..

> 
> What a snapshot actually does is create another reference (reflink)
> to an 
> extent.
[snip snap]
> So in the 
> case of nocow, a cow1 (one-time-cow) exception must be made,
> rewriting 
> the changed data to a new location, as the old location continues to
> be 
> referenced by at least one other reflink.
That's what I've meant.


> So (with the fact that writable snapshots are available and thus it
> can 
> be the snapshot that changed if it's what was written to) the one
> that 
> gets the changed fragment written elsewhere, thus getting fragmented,
> is 
> the one that changed, whether that's the working copy or the snapshot
> of 
> that working copy.
Yep,.. that's what I've suspected and asked for.

The "original" file, in the sense of the file that first reflinked the
contiguous blocks,... will continue to point to these continuous
blocks.

While the "new" file, i.e he CoW-1-ed snapshot's file, will partially
reflink blocks form the contiguous range, and it's rewritten blocks
will reflink somewhere else.
Thus the "new" file is the one that gets fragmented.


> > And one more:
> > You both said, auto-defrag is generally recommended.
> > Does that also apply for SSDs (where we want to avoid unnecessary
> > writes)?
> > It does seem to get enabled, when SSD mode is detected.
> > What would it actually do on an SSD?
> Did you mean it does _not_ seem to get (automatically) enabled, when
> SSD 
> mode is detected, or that it _does_ seem to get enabled, when 
> specifically included in the mount options, even on SSDs?
I does seem to get enabled, when specifically included in the mount
options (the ssd mount option is not used), i.e.:
/dev/mapper/system  /   btrfs   
subvol=/root,defaults,noatime,autodefrag0   1
leads to:
[5.294205] BTRFS: device label foo devid 1 transid 13 /dev/disk/by-label/foo
[5.295957] BTRFS info (device sdb3): disk space caching is enabled
[5.296034] BTRFS: has skinny extents
[   67.082702] BTRFS: device label system devid 1 transid 60710 
/dev/mapper/system
[   67.85] BTRFS info (device dm-0): disk space caching is enabled
[   67.111267] BTRFS: has skinny extents
[   67.305084] BTRFS: detected SSD devices, enabling SSD mode
[   68.562084] BTRFS info (device dm-0): enabling auto defrag
[   68.562150] BTRFS info (device dm-0): disk space caching is enabled



> Or did you actually mean it the way you wrote it, that it seems to be
> enabled (implying automatically, along with ssd), when ssd mode is 
> detected?
No, sorry for being unclear.
I meant it that way, that having the ssd detected doesn't auto-disable
auto-defrag, which I thought may make sense, given that I didn't know
exactly what it would do on SSDs...
IIRC, Hugo or Austin, mentioned the thing with making for better IOPS,
but I haven't had considered that to have impact enough... so I thought
it could have made sense to ignore the "autodefrag" mount option in
case an ssd was detected.



> There are three factors I'm aware of here as well, all favoring 
> autodefrag, just as the two above favored leaving it off.
> 
> 1) IOPS, Input/Output Operations Per Second.  SSDs typically have
> both an 
> IOPS and a throughput rating.  And unlike spinning rust, where raw
> non-
> sequential-write IOPS are generally bottlenecked by seek times, on
> SSDs 
> with their zero seek-times, IOPS can actually be the bottleneck.
Hmm it would be really nice to get someone who has found a way to make
some sound analysis/benchmarking of that.


> 2) SSD physical write and erase block sizes as multiples of the
> logical/
> read block size.  To the extent that extent sizes are multiples of
> the 
> write and/or erase-block size, writing larger extents will reduce
> write 
> amplification due to writing and blocks smaller than the write or
> erase 
> block size.
Hmm... okay I don't know the details of how btrfs does this, but I'd
have expected that all extents are aligned to the underlying physical
devices' block structure.
Thus each extent should start at such write/erase block, and at most it
shouldn't perfectly at the end of the extent.
If the file is fragmented (i.e. more than one extent), I'd have even
hoped that all but the last one fit perfectly.

So what you basically mean, AFAIU, is that by having auto-defrag, you
get larger extents (i.e. smaller ones collapsed into one) and by thus
you get less cut off at the end of extents where these 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Kai Krakow]

Am Wed, 9 Dec 2015 13:36:01 + (UTC)
schrieb Duncan <1i5t5.dun...@cox.net>:

> >> > 4) Duncan mentioned that defrag (and I guess that's also for
> >> > auto- defrag) isn't ref-link aware...
> >> > Isn't that somehow a complete showstopper?  
> 
> >> It is, but the one attempt at dealing with it caused massive data
> >> corruption, and it was turned off again.  
> 
> IIRC, it wasn't data corruption so much, as massive scaling issues,
> to the point where defrag was entirely useless, as it could take a
> week or more for just one file.
> 
> So the decision was made that a non-reflink-aware defrag that
> actually worked in something like reasonable time even if it did
> break reflinks and thus increase space usage, was of more use than a
> defrag that basically didn't work at all, because it effectively took
> an eternity. After all, you can always decide not to run it if you're
> worried about the space effects it's going to have, but if it's going
> to take a week or more for just one file, you effectively don't have
> the choice to run it at all.
> 
> > So... does this mean that it's still planned to be implemented some
> > day or has it been given up forever?  
> 
> AFAIK it's still on the list.  And the scaling issues are better, but
> one big thing holding it up now is quota management.  Quotas never
> have worked correctly, but they were a big part (close to half, IIRC)
> of the original snapshot-aware-defrag scaling issues, and thus must
> be reliably working and in a generally stable state before a
> snapshot-aware-defrag can be coded to work with them.  And without
> that, it's only half a solution that would have to be redone when
> quotes stabilized anyway, so really, quota code /must/ be stabilized
> to the point that it's not a moving target, before reimplementing
> snapshot-aware-defrag makes any sense at all.
> 
> But even at that point, while snapshot-aware-defrag is still on the
> list, I'm not sure if it's ever going to be actually viable.  It may
> be that the scaling issues are just too big, and it simply can't be
> made to work both correctly and in anything approaching practical
> time.  Time will tell, of course, but until then...

I'd like to throw in an idea... Couldn't auto-defrag just be made "sort
of reflink-aware" in a very simple fashion: Just let it ignore extents
that are shared?

That way you can still enjoy it benefits in a mixed-mode scenario where
you are working with snapshots partly but other subvolumes are never
taken snapshots of.

Comments?

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 16 Dec 2015 22:59:01 +0100 as
excerpted:

>> And there very well might be such a tool... five or ten years down the
>> road when btrfs is much more mature and generally stabilized, well
>> beyond the "still maturing and stabilizing" status of the moment.

> Hmm let's hope btrfs isn't finished only when the next-gen default fs
> arrives ;^)

[Again, breaking into smaller point replies...]

Well, given the development history for both zfs and btrfs to date, five 
to ten years down the line, with yet another even newer filesystem then 
already under development, is more being "real", than not.  Also see the 
history in MS' attempt at a next-gen filesystem.  The reality is these 
things take FAR longer than one might think.

FWIW, on the wiki I see feature points and benchmarks for v0.14, 
introduced in April of 2008, and a link to an earlier btree filesystem on 
which btrfs apparently was based, dating to 2006, so while I don't have a 
precise beginning date, and to some extent such a thing would be rather 
arbitrary anyway, as Chris would certainly have done some major thinking, 
preliminary research and coding, before his first announcement, a project 
origin in late 2006 or sometime in 2007 has to be quite close.

And (as I noted in a parenthetical at my discovery in a different 
thread), I switched to btrfs for my main filesystems when I bought my 
first SSDs, in June of 2013, so already a quarter decade ago.  At the 
time btrfs was just starting to remove some of the more dire 
"experimental" warnings.  Obviously it has stabilized quite a bit since 
then, but due to the oft-quoted 80/20 rule and extensions, where the last 
20% of the progress takes 80% of the work, etc...

It could well be another five years before btrfs is at a point I think 
most here would call stable.  That would be 2020 or so, about 13 years 
for the project, and if you look at the similar projects mentioned above, 
that really isn't unrealistic at all.  Ten years minimum, and that's with 
serious corporate level commitments and a lot more dedicated devs than 
btrfs has.  12 years not unusual at all, and a decade and a half still 
well within reasonable range, for a filesystem with this level of 
complexity, scope, and features.

And realistically, by that time, yet another successor filesystem may 
indeed be in the early stages of development, say at the 20/80 point, 20% 
of required effort invested, possibly 80% of the features done, but not 
stabilized.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 16 Dec 2015 22:59:01 +0100 as
excerpted:

> In kinda curios, what free space fragmentation actually means here.
> 
> Ist simply like this:
> +--+-+---++
> |     F    |  D  | F |    D   |
> +--+-+---++
> Where D is data (i.e. files/metadata) and F is free space.
> In other words, (F)ree space itself is not further subdivided and only
> fragmented by the (D)ata extents in between.
> 
> Or is it more complex like this:
> +-++-+---++
> |  F  |  F |  D  | F |    D   |
> +-++-+---++
> Where the (F)ree space itself is subdivided into "extents" (not
> necessarily of the same size), and btrfs couldn't use e.g. the first two
> F's as one contiguous amount of free space for a larger (D)ata extent

[still breaking into smaller points for reply]

At the one level, I had the simpler f/d/f/d scheme in mind, but that 
would be the case inside a single data chunk.  At the higher file level, 
with files significant fractions of the size of a single data chunk to 
much larger than a single data chunk, the more complex and second
f/f/d/f/d case would apply, with the chunk boundary as the separation 
between the f/f.

IOW, files larger than data chunk size will always be fragmented into 
data chunk size fragments/extents, at the largest, because chunks are 
designed to be movable using balance, device remove, replace, etc.

So (using the size numbers from a recent comment from Qu in a different 
thread), on a filesystem with under 100 GiB total space-effective (space-
effective, space available, accounting for the replication type, raid1, 
etc, and I'm simplifying here...), data chunks should be 1 GiB, while 
above that, with striping, they might be upto 10 GiB.

Using the 1 GiB nominal figure, files over 1 GiB would always be broken 
into 1 GiB maximum size extents, corresponding to 1 extent per chunk.

But while 4 KiB extents are clearly tiny and inefficient at today's 
scale, in practice, efficiency gains break down at well under GiB scale, 
with AFAIK 128 MiB being the upper bound at which any efficiency gains 
could really be expected, and 1 MiB arguably being a reasonable point at 
which further increases in extent size likely won't have a whole lot of 
effect even on SSD erase-block (where 1 MiB is a nominal max), but that's 
that's still 256X the usual 4 KiB minimum data block size, 8X the 128 KiB 
btrfs compression-block size, and 4X the 256 KiB defrag default "don't 
bother with extents larger than this" size.

Basically, the 256 KiB btrfs defrag "don't bother with anything larger 
than this" default is quite reasonable, tho for massive multi-gig VM 
images, the number of 256 KiB fragments will still look pretty big, so 
while technically a very reasonable choice, the "eye appeal" still isn't 
that great.

But based on real reports posting before and after numbers from filefrag 
(on uncompressed btrfs), we do have cases where defrag can't find 256 KiB 
free-space blocks and thus can actually fragment a file worse than it was 
before, so free-space fragmentation is indeed a very real problem.

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 16 Dec 2015 22:59:01 +0100 as
excerpted:

> I'm a bit unsure how to read filefrag's output... (even in the
> uncompressed case).
> What would it show me if there was fragmentation

/path/to/file:  18 extents found

It tells you the number of extents found.  Nominally, each extent should 
be a fragment, but as has been discussed elsewhere, on btrfs compressed 
files it will interpret each 128 KiB btrfs compression block as its own 
extent, even if (as seen in verbose mode) the next one begins where the 
previous one ends so it's really just a single extent.

Apparently on ext3/4, it's possible to have multi-gig files as a single 
extent, thus unfragmented, but as explained in an earlier reply to a 
point earlier in your post, on btrfs, extents of a GiB are nominally the 
best you can do as that's the nominal data chunk size, tho in limited 
circumstances larger extents are still possible on btrfs.

In the case above, where I took the 18 extents result from a real file 
(tho obviously the posted path isn't real), it was 4 MiB in size (I think 
exactly, it's a 4 MiB BIOS image =:^), so doing the math, extents average 
227 KiB.  That's on a filesystem that is always mounted with autodefrag, 
but it's also always mounted with compress, so it's possible some of the 
reported extents are compressed.

Actually, looking at filefrag -v output (which I've never used before but 
which someone noted could be used to check fragmentation on compressed 
files, tho it's not as straightforward as you might think), it looks like 
all but two of the listed extents are 32 blocks long (with 4096 byte 
blocks), which equates to 128 KiB, the btrfs compression-block size, and 
the two remaining extents are 224 blocks long or 896 KiB, an exact 7 
multiple of 128 KiB, so this file would indeed appear to be compressed 
except for those two uncompressed extents.  (As for figuring out how to 
interpret the full -v output to know whether the compressed blocks are 
actually single extents or not, as I said this is my first time trying
-v, and I didn't bother going that far with it.)

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 16 Dec 2015 22:59:01 +0100 as
excerpted:

>> he obviously didn't think thru the fact that compression MUST be a
>> rewrite, thereby breaking snapshot reflinks, even were normal
>> non-compression defrag to be snapshot aware, because compression
>> substantially changes the way the file is stored), that's _implied_,
>> not explicit.
> So you mean, even if ref-link aware defrag would return, it would still
> break them again when compressing/uncompressing/recompressing?
> I'd have hoped that then, all snapshots respectively other reflinks
> would simply also change to being compressed,

You're correct.  I "obviously didn't thing thru" that the whole way, 
myself. =:^(

But meanwhile, we don't have snapshot-aware-defrag, and in that case, the 
implication... and his result... remains.

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 16 Dec 2015 22:59:01 +0100 as
excerpted:

>> It's certainly in quite a few on-list posts over the years

> okay,.. in other words: no ;-)
> scatter over the years list posts don't count as documentation :P

=:^)

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 16 Dec 2015 22:59:01 +0100 as
excerpted:

> On Wed, 2015-12-09 at 16:36 +, Duncan wrote:
>> But... as I've pointed out in other replies, in many cases including
>> this specific one (bittorrent), applications have already had to
>> develop their own integrity management features

> Well let's move discussion upon that into the "dear developers, can we
> have notdatacow + checksumming, plz?" where I showed in one of the more
> recent threads that bittorrent seems rather to be the only thing which
> does use that per default... while on the VM image front, nothing seems
> to support it, and on the DB front, some support it, but don't use it
> per default.
> 
>> In the bittorrent case specifically, torrent chunks are already
>> checksummed, and if they don't verify upon download, the chunk is
>> thrown away and redownloaded.

> I'm not a bittorrent expert, because I don't use it, but that sounds to
> be more like the edonkey model, where - while there are checksums -
> these are only used until the download completes. Then you have the
> complete file, any checksum info thrown away, and the file again being
> "at risk" (i.e. not checksum protected).

[I'm breaking this into smaller replies again.]

Just to mention here, that I said "integrity management features", which 
includes more than checksumming.  As Austin Hemmelgarn has been pointing 
out, DBs and some VMs do COW, some DBs do checksumming or at least have 
that option, and both VMs and DBs generally do at least some level of 
consistency checking as they load.  Those are all "integrity management 
features" at some level.

As for bittorrent, I /think/ the checksums are in the torrent files 
themselves (and if I'm not mistaken, much as git, the chunks within the 
file are actually IDed by checksum, not specific position, so as long as 
the torrent is active, uploading or downloading, these will by definition 
be retained).  As long as those are retained, the checksums should be 
retained.  And ideally, people will continue to torrent the files long 
after they've finished downloading them, in which case they'll still need 
the torrent files themselves, along with the checksums info.

And for longer term storage, people really should be copying/moving their 
torrented files elsewhere, in such a way that they either eliminate the 
fragmentation if the files weren't nocowed, or eliminate the nocow 
attribute and get them checksum-protected as normal for files not 
intended to be constantly randomly rewritten, which will be the case once 
they're no longer being actively downloaded.  Of course that's at the 
slightly technically oriented user level, but then, the whole nocow 
thing, or even caring about checksums and longer term file integrity in 
the first place, is also technically oriented user level.  Normal users 
will just download without worrying about the nocow in the first place, 
and perhaps wonder why the disk is thrashing so, but not be inclined to 
do anything about it except perhaps switch back to their old filesystem, 
where it was faster and the disk didn't sound as bad.  In doing so, 
they'll either automatically get the checksuming along with the worse 
performance, or go back to a filesystem without the checksumming, and 
think it's fine as they know no different.

Meanwhile, if they do it correctly there's no window without protection, 
as the torrent file can be used to double-verify the file once moved, as 
well, before deleting it.

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Mon, 14 Dec 2015 02:44:55 +0100 as
excerpted:

> Two more on these:
> 
> On Thu, 2015-11-26 at 00:33 +, Hugo Mills wrote:
>> 3) When I would actually disable datacow for e.g. a subvolume that
>> > holds VMs or DBs... what are all the implications?

>> After snapshotting, modifications are CoWed precisely once, and
>> then it reverts to nodatacow again. This means that making a snapshot
>> of a nodatacow object will cause it to fragment as writes are made to
>> it.

> AFAIU, the one the get's fragmented then is the snapshot, right, and the
> "original" will stay in place where it was? (Which is of course good,
> because one probably marked it nodatacow, to avoid that fragmentation
> problem on internal writes).

No.  Or more precisely, keep in mind that from btrfs' perspective, in 
terms of reflinks, once made, there's no "original" in terms of special 
treatment, all references to the extent are treated the same.

What a snapshot actually does is create another reference (reflink) to an 
extent.  What btrfs normally does on change as a cow-based filesystem is 
of course copy-on-write the change.  What nocow does, in the absence of 
other references to that extent, is rewrite the change in-place.

But if there's another reference to that extent, the change can't be in-
place because that would change the file reached by that other reference 
as well, and the change was only to be made to one of them.  So in the 
case of nocow, a cow1 (one-time-cow) exception must be made, rewriting 
the changed data to a new location, as the old location continues to be 
referenced by at least one other reflink.

So (with the fact that writable snapshots are available and thus it can 
be the snapshot that changed if it's what was written to) the one that 
gets the changed fragment written elsewhere, thus getting fragmented, is 
the one that changed, whether that's the working copy or the snapshot of 
that working copy.

> I'd assume the same happens when I do a reflink cp.

Yes.  It's the same reflinking mechanism, after all.  If there's other 
reflinks to the extent, snapshot or otherwise, changes must be written 
elsewhere, even if they'd otherwise be nocow.

> Can one make a copy, where one still has atomicity (which I guess
> implies CoW) but where the destination file isn't heavily fragmented
> afterwards,... i.e. there's some pre-allocation, and then cp really does
> copy each block (just everything's at the state of time where I stared
> cp, not including any other internal changes made on the source in
> between).

The way that's handled is via ro snapshots which are then copied, which 
of course is what btrfs send does (at least in non-incremental mode, and 
incremental mode still uses the ro snapshot part to get atomicity), in 
effect.

> And one more:
> You both said, auto-defrag is generally recommended.
> Does that also apply for SSDs (where we want to avoid unnecessary
> writes)?
> It does seem to get enabled, when SSD mode is detected.
> What would it actually do on an SSD?

Did you mean it does _not_ seem to get (automatically) enabled, when SSD 
mode is detected, or that it _does_ seem to get enabled, when 
specifically included in the mount options, even on SSDs?

Or did you actually mean it the way you wrote it, that it seems to be 
enabled (implying automatically, along with ssd), when ssd mode is 
detected?

Because the latter would be a shock to me, as that behavior hasn't been 
documented anywhere, but I can't imagine it's actually doing it and that 
you actually meant what you actually wrote.


If you look waaayyy back to shortly before I did my first more or less 
permanent deployment (I had initially posted some questions and did an 
initial experimental deployment several months earlier, but it didn't 
last long, because $reasons), you'll see a post I made to the list with 
pretty much the same general question, autodefrag on ssd, or not.

I believe the most accurate short answer is that the benefit of 
autodefrag on SSD is fuzzy, and thus left to local choice/policy, without 
an official recommendation either way.

There are two points that we know for certain: (1) the zero-seek-time of 
SSD effectively nullifies the biggest and most direct cost associated 
with fragmentation on spinning rust, thereby lessening the advantage of 
autodefrag as seen on spinning rust by an equally large degree, and (2) 
autodefrag will without question lead to a relatively limited number of 
near-time additional writes, as the rewrite is queued and eventually 
processed.

To the extent that an admin considers these undisputed factors alone, or 
weighs them less heavily than the more controversial factors below, 
they're likely to consider autodefrag on ssd a net negative and leave it 
off.

But I was persuaded by the discussion when I asked the question, to 
enable autodefrag on my all-ssd btrfs deployment here.  Why?  Those 
other, less direct and arguably less directly 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Mon, 14 Dec 2015 03:46:01 +0100 as
excerpted:

>> Same here.  In fact, my most anticipated feature is N-way-mirroring,
> Hmm ... not totally sure about that...
> AFAIU, N-way-mirroring is what currently the currently wrongly called
> RAID1 is in btrfs, i.e. having N replicas of everything on M devices,
> right?
> In other words, not being a N-parity-RAID and not guaranteeing that
> *any* N disks could fail, right?

No.  N-way-mirroring, at least in simplest form (as in md/raid1) is N 
replicas on N devices, so loss of N-1 devices is permitted without loss 
of data.

Normally the best thing about this is that unlike parity, once the 
general support is in, you can increase redundancy at will, with 
guaranteed device-loss protection of as many devices as you care to 
insure against.

At one point with somewhat old devices that I didn't particularly trust 
any more and because I had them from a previous raid6 setup, I was 
running 4-way-md/raid1.

Of course with md/raid1, the problem is lack of any sort of data 
integrity assurance, even scrubbing just arbitrarily chooses one and in 
the case of difference, simply copies that to the others, not even 
plurality-vote most authoritative version.

With btrfs checksumming, the value of N-way-mirroring is increased 
dramatically, since it allows individual block verification and fallback, 
as opposed to whole-device-loss.

While my own sweet-spot balance will tend to be three-way, avoiding the 
"if one copy is bad (perhaps because of a device that's known failing/
failed), you better /hope/ your only remaining copy is good" problem of 
the present two-way-only solution, I could easily see people finding 
value in 4/5/6-way mirroring as well.

And of course if that is extended to raid10, three-way-mirroring, two-way-
striping, on six total devices, would be my preferred, over the three-way-
striped, two-way-mirrored, that's the only current choice for six-device 
btrfs raid10.

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Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

On Wed, 2015-12-09 at 13:36 +, Duncan wrote:
> Answering the BTW first, not to my knowledge, and I'd be
> skeptical.  In 
> general, btrfs is cowed, and that's the focus.  To the extent that
> nocow 
> is necessary for fragmentation/performance reasons, etc, the idea is
> to 
> try to make cow work better in those cases, for example by working on
> autodefrag to make it better at handling large files without the
> scaling 
> issues it currently has above half a gig or so, and thus to confine
> nocow 
> to a smaller and smaller niche use-case, rather than focusing on
> making 
> nocow better.
> Of course it remains to be seen how much better they can do with 
> autodefrag, etc, but at this point, there's way more project 
> possibilities than people to develop them, so even if they do find
> they 
> can't make cow work much better for these cases, actually working on
> nocow 
> would still be rather far down the list, because there's so many
> other 
> improvement and feature opportunities that will get the focus
> first.  
> Which in practice probably puts it in "it'd be nice, but it's low
> enough 
> priority that we're talking five years out or more, unless of course 
> someone else qualified steps up and that's their personal itch they
> want 
> to scratch", territory.
I guess I'll split out my answer on that, in a fresh thread about
checksums for nodatacow later, hoping to attract some more devs there
:-)

I think however, again with my naive understanding on how CoW works and
what it inherently implies, that there cannot be a real good solution
to the fragmentation problem for DB/etc. files.

And as such, I'd think that having a checksumming feature for
notdatacow as well, even if it's not perfect, is definitely worth it.


> As for the updated checksum after modification, the problem with that
> is 
> that in the mean time, the checksum wouldn't verify,
Well one could either implement some locking,.. but I don't see the
general problem here... if the block is still being written (and I
count updating the meta-data, including checksum, to that) it cannot be
read anyway, can it? It may be only half written and the data returned
would be garbage.


>  and while btrfs 
> could of course keep status in memory during normal operations,
> that's 
> not the problem, the problem is what happens if there's a crash and
> in-
> memory state vaporizes.  In that case, when btrfs remounted, it'd
> have no 
> way of knowing why the checksum didn't match, just that it didn't,
> and 
> would then refuse access to that block in the file, because for all
> it 
> knows, it /is/ a block error.
And this would only happen in the rare cases that anything crashes,
where it's anyway quite likely that this no-CoWed block will be
garbage.
I'll talk about that more in the separate thread... so let's move
things there.


> Same here.  In fact, my most anticipated feature is N-way-mirroring, 
Hmm ... not totally sure about that...
AFAIU, N-way-mirroring is what currently the currently wrongly called
RAID1 is in btrfs, i.e. having N replicas of everything on M devices,
right?
In other words, not being a N-parity-RAID and not guaranteeing that
*any* N disks could fail, right?

Hmm I guess that would be definitely nice to have, especially since
then we could have true RAID1, i.e. N=M.

But it's probably rather important for those scenarios, where either
resilience matters a lot... and/or   those where write speed doesn't
but read speed does, right?

Taking the example of our use case at the university, i.e. the LHC
Tier-2 we run,... that would rather be uninteresting.
We typically have storage nodes (and many of them) of say 16-24
devices, and based on funding constraints, resilience concerns and IO
performance, we place them in RAID6 (yeah i know, RAID5 is faster, but
even with hotspares in place, practise lead too often to lost RAIDs).

Especially for the bigger nodes, with more disks, we'd rather have a N-
parity RAID, where any N disks can fail)... of course performance
considerations may kill that desire again ;)


> It is a big and basic feature, but turning it off isn't the end of
> the 
> world, because then it's still the same level of reliability other 
> solutions such as raid generally provide.
Sure... I never meant it as "loss to what we already have in other
systems"... but as "loss compared to how awesome[0] btrfs could be ;-)"


> But as it happens, both VM image management and databases tend to
> come 
> with their own integrity management, in part precisely because the 
> filesystem could never provide that sort of service.
Well that's only partially true, to my knowledge.
a) I wouldn't know that hypervisors do that at all.
b) DBs have of course their journal, but that protects only against
crashes,... not against bad blocks nor does it help you to decide which
block is good when you have multiple.


> After all, you can always decide not to run it if you're worried
> about the space effects it's going to have

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

Two more on these:

On Thu, 2015-11-26 at 00:33 +, Hugo Mills wrote:
> 3) When I would actually disable datacow for e.g. a subvolume that
> > holds VMs or DBs... what are all the implications?
> > Obviously no checksumming, but what happens if I snapshot such a
> > subvolume or if I send/receive it?
>    After snapshotting, modifications are CoWed precisely once, and
> then it reverts to nodatacow again. This means that making a snapshot
> of a nodatacow object will cause it to fragment as writes are made to
> it.
AFAIU, the one the get's fragmented then is the snapshot, right, and
the "original" will stay in place where it was? (Which is of course
good, because one probably marked it nodatacow, to avoid that
fragmentation problem on internal writes).

I'd assume the same happens when I do a reflink cp.

Can one make a copy, where one still has atomicity (which I guess
implies CoW) but where the destination file isn't heavily fragmented
afterwards,... i.e. there's some pre-allocation, and then cp really
does copy each block (just everything's at the state of time where I
stared cp, not including any other internal changes made on the source
in between).


And one more:
You both said, auto-defrag is generally recommended.
Does that also apply for SSDs (where we want to avoid unnecessary
writes)?
It does seem to get enabled, when SSD mode is detected.
What would it actually do on an SSD?


Cheers,
Chris.

smime.p7s
Description: S/MIME cryptographic signature

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 09 Dec 2015 06:43:01 +0100 as
excerpted:

> Hey Hugo,
> 
> 
> On Thu, 2015-11-26 at 00:33 +, Hugo Mills wrote:
> 
>> The issue is that nodatacow bypasses the transactional nature of
>> the FS, making changes to live data immediately. This then means that
>> if you modify a modatacow file, the csum for that modified section is
>> out of date, and won't be back in sync again until the latest
>> transaction is committed. So you can end up with an inconsistent
>> filesystem if there's a crash between the two events.

> Sure,... (and btw: is there some kind of journal planned for
> nodatacow'ed files?),... but why not simply trying to write an updated
> checksum after the modified section has been flushed to disk... of
> course there's no guarantee that both are consistent in case of crash (
> but that's also the case without any checksum)... but at least one would
> have the csum protection against everything else (blockerrors and that
> like) in case no crash occurs?

Answering the BTW first, not to my knowledge, and I'd be skeptical.  In 
general, btrfs is cowed, and that's the focus.  To the extent that nocow 
is necessary for fragmentation/performance reasons, etc, the idea is to 
try to make cow work better in those cases, for example by working on 
autodefrag to make it better at handling large files without the scaling 
issues it currently has above half a gig or so, and thus to confine nocow 
to a smaller and smaller niche use-case, rather than focusing on making 
nocow better.

Of course it remains to be seen how much better they can do with 
autodefrag, etc, but at this point, there's way more project 
possibilities than people to develop them, so even if they do find they 
can't make cow work much better for these cases, actually working on nocow 
would still be rather far down the list, because there's so many other 
improvement and feature opportunities that will get the focus first.  
Which in practice probably puts it in "it'd be nice, but it's low enough 
priority that we're talking five years out or more, unless of course 
someone else qualified steps up and that's their personal itch they want 
to scratch", territory.

As for the updated checksum after modification, the problem with that is 
that in the mean time, the checksum wouldn't verify, and while btrfs 
could of course keep status in memory during normal operations, that's 
not the problem, the problem is what happens if there's a crash and in-
memory state vaporizes.  In that case, when btrfs remounted, it'd have no 
way of knowing why the checksum didn't match, just that it didn't, and 
would then refuse access to that block in the file, because for all it 
knows, it /is/ a block error.

And there's already a mechanism for telling btrfs to ignore checksums, 
and nocow already activates it, so... there's really nothing more to be 
done.

>> > For me the checksumming is actually the most important part of btrfs
>> > (not that I wouldn't like its other features as well)... so turning
>> > it off is something I really would want to avoid.

Same here.  In fact, my most anticipated feature is N-way-mirroring, 
since that will allow three copies (or more, but three is my sweet spot 
balance between the space and reliability factors) instead of the current 
limit of two.  It just disturbs me than in the event of one copy being 
bad, the other copy /better/ be good, because there's no further 
fallback!  With a third copy, there'd be that one further fallback, and 
the chances of all three copies failing checksum verification are remote 
enough I'm willing to risk it, given the incremental cost of additional 
copies.

>> > Plus it opens questions like: When there are no checksums, how can it
>> > (in the RAID cases) decide which block is the good one in case of
>> > corruptions?

>>    It doesn't decide -- both copies look equally good, because
>> there's no checksum, so if you read the data, the FS will return
>> whatever data was on the copy it happened to pick.

> Hmm I see... so one gets basically the behaviour of RAID.
> Isn't that kind of a big loss? I always considered the guarantee against
> block errors and that like one of the big and basic features of btrfs.

It is a big and basic feature, but turning it off isn't the end of the 
world, because then it's still the same level of reliability other 
solutions such as raid generally provide.

And the choice to turn it off is just that, a choice, tho it's currently 
the recommended one in some cases, such as with large VM images, etc.

But as it happens, both VM image management and databases tend to come 
with their own integrity management, in part precisely because the 
filesystem could never provide that sort of service.  So to the extent 
that btrfs must turn off its integrity management features when dealing 
with that sort of file, it's no bigger deal than it would be on any other 
filesystem, it's simply returning what's normally a 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Wed, 09 Dec 2015 06:45:47 +0100 as
excerpted:

> On 2015-11-27 00:08, Duncan wrote:
>> Christoph Anton Mitterer posted on Thu, 26 Nov 2015 01:23:59 +0100 as
>> excerpted:
>>> 1) AFAIU, the fragmentation problem exists especially for those files
>>> that see many random writes, especially, but not limited to, big
>>> files. Now that databases and VMs are affected by this, is probably
>>> broadly known in the meantime (well at least by people on that list).
>>> But I'd guess there are n other cases where such IO patterns can
>>> happen which one simply never notices, while the btrfs continues to
>>> degrade.
>> 
>> The two other known cases are:
>> 
>> 1) Bittorrent download files, where the full file size is preallocated
>> (and I think fsynced), then the torrent client downloads into it a
>> chunk at a time.

> Okay, sounds obvious.
> 
>> The more general case would be any time a file of some size is
>> preallocated and then written into more or less randomly, the problem
>> being the preallocation, which on traditional rewrite-in-place
>> filesystems helps avoid fragmentation (as well as ensuring space to
>> save the full file), but on COW-based filesystems like btrfs, triggers
>> exactly the fragmentation it was trying to avoid.

> Is it really just the case when the file storage *is* actually fully
> pre-allocated?
> Cause that wouldn't (necessarily) be the case for e.g. VM images (e.g.
> qcow2, or raw images when these are sparse files).
> Or is it rather any case where, in larger file, many random (file
> internal) writes occur?

It's the second case, or rather, the reverse of the first case, since 
preallocation and fsync, then write into it, is one specific subset case 
of the broader case of random rewrites into existing files.  VM images 
and database files are two other specific subset cases of the same 
broader case superset.

>> arranging to have the client write into a dir with the nocow attribute
>> set, so newly created torrent files inherit it and do rewrite-in-place,
>> is highly recommended.

> At the IMHO pretty high expense of loosing the checksumming :-(
> Basically loosing half of the main functionalities that make btrfs
> interesting for me.

But... as I've pointed out in other replies, in many cases including this 
specific one (bittorrent), applications have already had to develop their 
own integrity management features, because other filesystems didn't 
supply them and the apps simply didn't work reliably without those 
features.

In the bittorrent case specifically, torrent chunks are already 
checksummed, and if they don't verify upon download, the chunk is thrown 
away and redownloaded.

And after the download is complete and the file isn't being constantly 
rewritten, it's perfectly fine to copy it elsewhere, into a dir where 
nocow doesn't apply.  With the copy, btrfs will create checksums, and if 
you're paranoid you can hashcheck the original nocow copy against the new 
checksummed/cow copy, and after that, any on-media changes will be caught 
by the normal checksum verification mechanisms.

Further, at least some bittorrent clients make preallocation an option.  
Here, on btrfs I'd simply turn off that option, rather than bothering 
with nocow in the first place.  That should already reduce fragmentation 
significantly due to the 30-second by default commit frequency, tho there 
will likely still be some fragmentation due to the out-of-order 
downloading.  But either autodefrag or the previously mentioned post-
download recopy should deal with that.

> For databases, will e.g. the vacuuming maintenance tasks solve the
> fragmentation issues (cause I guess at least when doing full vacuuming,
> it will rewrite the files).

If it does full rewrite, it should, provided the freespace itself isn't 
so fragmented that it's impossible to find sufficiently large extents to 
avoid fragmentation.

Of course there's also autodefrag, if the database isn't so busy and/or 
the database files are small enough that the defragging rewrites don't 
trigger bottlenecking, the primary downside risk with autodefrag.

>> The problem is much reduced in newer systemd, which is btrfs aware and
>> in fact uses btrfs-specific features such as subvolumes in a number of
>> cases (creating subvolumes rather than directories where it makes sense
>> in some shipped tmpfiles.d config files, for instance), if it's running
>> on btrfs.

> Hmm doesn't seem really good to me if systemd would do that, cause it
> then excludes any such files from being snapshot.

Of course if the directories are already present due to systemd upgrading 
from non-btrfs-aware versions, they'll remain as normal dirs, not 
subvolumes.  This is the case here.

And of course you can switch them around to dirs if you like, and/or 
override the shipped tmpfiles.d config with your own.

Meanwhile, distros that both ship systemd and offer btrfs as a filesystem 
option (or use it by default), should 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

Hey Hugo,


On Thu, 2015-11-26 at 00:33 +, Hugo Mills wrote:
>    Answering the second part first, no, it can't.
Thanks so far :)


>    The issue is that nodatacow bypasses the transactional nature of
> the FS, making changes to live data immediately. This then means that
> if you modify a modatacow file, the csum for that modified section is
> out of date, and won't be back in sync again until the latest
> transaction is committed. So you can end up with an inconsistent
> filesystem if there's a crash between the two events.
Sure,... (and btw: is there some kind of journal planned for
nodatacow'ed files?),... but why not simply trying to write an updated
checksum after the modified section has been flushed to disk... of
course there's no guarantee that both are consistent in case of crash (
but that's also the case without any checksum)... but at least one
would have the csum protection against everything else (blockerrors and
that like) in case no crash occurs?



> > For me the checksumming is actually the most important part of
> > btrfs
> > (not that I wouldn't like its other features as well)... so turning
> > it
> > off is something I really would want to avoid.
> > 
> > Plus it opens questions like: When there are no checksums, how can
> > it
> > (in the RAID cases) decide which block is the good one in case of
> > corruptions?
>    It doesn't decide -- both copies look equally good, because
> there's
> no checksum, so if you read the data, the FS will return whatever
> data
> was on the copy it happened to pick.
Hmm I see... so one gets basically the behaviour of RAID.
Isn't that kind of a big loss? I always considered the guarantee
against block errors and that like one of the big and basic features of
btrfs.
It seems that for certain (not too unimportant cases: DBs, VMs) one has
to decide between either evil, loosing the guaranteed consistency via
checksums... or basically running into severe troubles (like Mitch's
reported fragmentation issues).


> > 3) When I would actually disable datacow for e.g. a subvolume that
> > holds VMs or DBs... what are all the implications?
> > Obviously no checksumming, but what happens if I snapshot such a
> > subvolume or if I send/receive it?
> 
>    After snapshotting, modifications are CoWed precisely once, and
> then it reverts to nodatacow again. This means that making a snapshot
> of a nodatacow object will cause it to fragment as writes are made to
> it.
I see... something that should possibly go to some advanced admin
documentation (if not already in).
It means basically, that one must assure that any such files (VM
images, DB data dirs) are already created with nodatacow (perhaps on a
subvolume which is mounted as such.


> > 4) Duncan mentioned that defrag (and I guess that's also for auto-
> > defrag) isn't ref-link aware...
> > Isn't that somehow a complete showstopper?
>    It is, but the one attempt at dealing with it caused massive data
> corruption, and it was turned off again.
So... does this mean that it's still planned to be implemented some day
or has it been given up forever?
And is it (hopefully) also planned to be implemented for reflinks when
compression is added/changed/removed?


Given that you (or Duncan?,... sorry I sometimes mix up which of said
exactly what, since both of you are notoriously helpful :-) ) mentioned
that autodefrag basically fails with larger files,... and given that it
seems to be quite important for btrfs to not be fragmented too heavily,
it sounds a bit as if anything that uses (multiple) reflinks (e.g.
snapshots) cannot be really used very well.


>  autodefrag, however, has
> always been snapshot aware and snapshot safe, and would be the
> recommended approach here.
Ahhh... so autodefag *is* snapshot aware, and that's basically why the
suggestion is (AFAIU) that it's turned on, right?
So, I'm afraid O:-), that triggers a follow-up question:
Why isn't it the default? Or in other words what are its drawbacks
(e.g. other cases where ref-links would be broken up,... or issues with
compression)?

And also, when I now activate it on an already populated fs, will it
defrag also any old files (even if they're not rewritten or so)?
I tried to have a look for some general (rather "for dummies" than for
core developers) description of how defrag and autodefrag work... but
couldn't find anything in the usual places... :-(

btw: The wiki (https://btrfs.wiki.kernel.org/index.php/UseCases#How_do_
I_defragment_many_files.3F) doesn't mention that auto-defrag doesn't
suffer from that problem.


>  (Actually, it was broken in the same
> incident I just described -- but fixed again when the broken patches
> were reverted).
So it just couldn't be fixed (hopfully: yet) for the (manual) online
defragmentation?!


> > 5) Especially keeping (4) in mind but also the other comments in
> > from
> > Duncan and Austin...
> > Is auto-defrag now recommended to be generally used?
>
>    Absolutely, yes.
I see... well, I'll probably wait 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

On 2015-11-27 00:08, Duncan wrote:
> Christoph Anton Mitterer posted on Thu, 26 Nov 2015 01:23:59 +0100 as
> excerpted:
>> 1) AFAIU, the fragmentation problem exists especially for those files
>> that see many random writes, especially, but not limited to, big files.
>> Now that databases and VMs are affected by this, is probably broadly
>> known in the meantime (well at least by people on that list).
>> But I'd guess there are n other cases where such IO patterns can happen
>> which one simply never notices, while the btrfs continues to degrade.
> 
> The two other known cases are:
> 
> 1) Bittorrent download files, where the full file size is preallocated 
> (and I think fsynced), then the torrent client downloads into it a chunk 
> at a time.
Okay, sounds obvious.


> The more general case would be any time a file of some size is 
> preallocated and then written into more or less randomly, the problem 
> being the preallocation, which on traditional rewrite-in-place 
> filesystems helps avoid fragmentation (as well as ensuring space to save 
> the full file), but on COW-based filesystems like btrfs, triggers exactly 
> the fragmentation it was trying to avoid.
Is it really just the case when the file storage *is* actually fully
pre-allocated?
Cause that wouldn't (necessarily) be the case for e.g. VM images (e.g.
qcow2, or raw images when these are sparse files).
Or is it rather any case where, in larger file, many random (file
internal) writes occur?


> arranging to 
> have the client write into a dir with the nocow attribute set, so newly 
> created torrent files inherit it and do rewrite-in-place, is highly 
> recommended.
At the IMHO pretty high expense of loosing the checksumming :-(
Basically loosing half of the main functionalities that make btrfs
interesting for me.


> It's also worth noting that once the download is complete, the files 
> aren't going to be rewritten any further, and thus can be moved out of 
> the nocow-set download dir and treated normally.
Sure... but this requires manual intervention.

For databases, will e.g. the vacuuming maintenance tasks solve the
fragmentation issues (cause I guess at least when doing full vacuuming,
it will rewrite the files).


> The problem is much reduced in newer systemd, which is btrfs aware and in 
> fact uses btrfs-specific features such as subvolumes in a number of cases 
> (creating subvolumes rather than directories where it makes sense in some 
> shipped tmpfiles.d config files, for instance), if it's running on 
> btrfs.
Hmm doesn't seem really good to me if systemd would do that, cause it
then excludes any such files from being snapshot.


> For the journal, I /think/ (see the next paragraph) that it now 
> sets the journal files nocow, and puts them in a dedicated subvolume so 
> snapshots of the parent won't snapshot the journals, thereby helping to 
> avoid the snapshot-triggered cow1 issue.
The same here, kinda disturbing if systemd would decide that on it's
own, i.e. excluding files from being checksum protected...


>> So is there any general approach towards this?
> The general case is that for normal desktop users, it doesn't tend to be 
> a problem, as they don't do either large VMs or large databases,
Well depends a bit on how one defines the "normal desktop user",... for
e.g. developers or more "power users" it's probably not so unlikely that
they do run local VMs for testing or whatever.

> and 
> small ones such as the sqlite files generated by firefox and various 
> email clients are handled quite well by autodefrag, with that general 
> desktop usage being its primary target.
Which is however not yet the default...


> For server usage and the more technically inclined workstation users who 
> are running VMs and larger databases, the general feeling seems to be 
> that those adminning such systems are, or should be, technically inclined 
> enough to do their research and know when measures such as nocow and 
> limited snapshotting along with manual defrags where necessary, are 
> called for.
mhh... well it's perhaps simple to expect that knowledge for few things
like VMs, DBs and that like... but there are countless of software
systems, many of them being more or less like a black box, at least with
respect to their internals.

It feels a bit, if there should be some tools provided by btrfs, which
tell the users which files are likely problematic and should be nodatacow'ed


> And if they don't originally, they find out when they start 
> researching why performance isn't what they expected and what to do about 
> it. =:^)
Which can take quite a while to be found out...


>> And what are the actual possible consequences? Is it just that fs gets
>> slower (due to the fragmentation) or may I even run into other issues to
>> the point the space is eaten up or the fs becomes basically unusable?
> It's primarily a performance issue, tho in severe cases it can also be a 
> scaling issue, to the point that maintenance tasks such 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Duncan]

Christoph Anton Mitterer posted on Thu, 26 Nov 2015 01:23:59 +0100 as
excerpted:

> Hey.
> 
> I've worried before about the topics Mitch has raised.
> Some questions.
> 
> 1) AFAIU, the fragmentation problem exists especially for those files
> that see many random writes, especially, but not limited to, big files.
> Now that databases and VMs are affected by this, is probably broadly
> known in the meantime (well at least by people on that list).
> But I'd guess there are n other cases where such IO patterns can happen
> which one simply never notices, while the btrfs continues to degrade.

The two other known cases are:

1) Bittorrent download files, where the full file size is preallocated 
(and I think fsynced), then the torrent client downloads into it a chunk 
at a time.

The more general case would be any time a file of some size is 
preallocated and then written into more or less randomly, the problem 
being the preallocation, which on traditional rewrite-in-place 
filesystems helps avoid fragmentation (as well as ensuring space to save 
the full file), but on COW-based filesystems like btrfs, triggers exactly 
the fragmentation it was trying to avoid.

At least some torrent clients (ktorrent at least) have an option to turn 
off that preallocation, however, and that would be recommended where 
possible.  Where disabling the preallocation isn't possible, arranging to 
have the client write into a dir with the nocow attribute set, so newly 
created torrent files inherit it and do rewrite-in-place, is highly 
recommended.

It's also worth noting that once the download is complete, the files 
aren't going to be rewritten any further, and thus can be moved out of 
the nocow-set download dir and treated normally.  For those who will 
continue to seed the files for some time, this could be done, provided 
the client can seed from a directory different than the download dir.

2) As a subcase of the database file case that people may not think 
about, systemd journal files are known to have had the internal-rewrite-
pattern problem in the past.  Apparently, while they're mostly append-
only in general, they do have an index at the beginning of the file that 
gets rewritten quite a bit.

The problem is much reduced in newer systemd, which is btrfs aware and in 
fact uses btrfs-specific features such as subvolumes in a number of cases 
(creating subvolumes rather than directories where it makes sense in some 
shipped tmpfiles.d config files, for instance), if it's running on 
btrfs.  For the journal, I /think/ (see the next paragraph) that it now 
sets the journal files nocow, and puts them in a dedicated subvolume so 
snapshots of the parent won't snapshot the journals, thereby helping to 
avoid the snapshot-triggered cow1 issue.

On my own systems, however, I've configured journald to only use the 
volatile tmpfs journals in /run, not the permanent /var location, 
tweaking the size of the tmpfs mounted on /run and the journald config so 
it normally stores a full boot session, but of course doesn't store 
journals from previous sessions as they're wiped along with the tmpfs at 
reboot.  I run syslog-ng as well, configured to work with journald, and 
thus have its more traditional append-only plain-text syslogs for 
previous boot sessions.

For my usage that actually seems the best of both worlds as I get journald 
benefits such as service status reports showing the last 10 log entries 
for that service, etc, with those benefits mostly applying to the current 
session only, while I still have the traditional plain-text greppable, 
etc, syslogs, from both the current and previous sessions, back as far as 
my log rotation policy keeps them.  It also keeps the journals entirely 
off of btrfs, so that's one particular problem I don't have to worry 
about at all, the reason I'm a bit fuzzy on the exact details of systemd's 
solution to the journal on btrfs issue.

> So is there any general approach towards this?

The general case is that for normal desktop users, it doesn't tend to be 
a problem, as they don't do either large VMs or large databases, and 
small ones such as the sqlite files generated by firefox and various 
email clients are handled quite well by autodefrag, with that general 
desktop usage being its primary target.

For server usage and the more technically inclined workstation users who 
are running VMs and larger databases, the general feeling seems to be 
that those adminning such systems are, or should be, technically inclined 
enough to do their research and know when measures such as nocow and 
limited snapshotting along with manual defrags where necessary, are 
called for.  And if they don't originally, they find out when they start 
researching why performance isn't what they expected and what to do about 
it. =:^)

> And what are the actual possible consequences? Is it just that fs gets
> slower (due to the fragmentation) or may I even run into other issues to
> the point the space is 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Hugo Mills]

On Thu, Nov 26, 2015 at 01:23:59AM +0100, Christoph Anton Mitterer wrote:
> 2) Why does notdatacow imply nodatasum and can that ever be decoupled?

   Answering the second part first, no, it can't.

   The issue is that nodatacow bypasses the transactional nature of
the FS, making changes to live data immediately. This then means that
if you modify a modatacow file, the csum for that modified section is
out of date, and won't be back in sync again until the latest
transaction is committed. So you can end up with an inconsistent
filesystem if there's a crash between the two events.

> For me the checksumming is actually the most important part of btrfs
> (not that I wouldn't like its other features as well)... so turning it
> off is something I really would want to avoid.
> 
> Plus it opens questions like: When there are no checksums, how can it
> (in the RAID cases) decide which block is the good one in case of
> corruptions?

   It doesn't decide -- both copies look equally good, because there's
no checksum, so if you read the data, the FS will return whatever data
was on the copy it happened to pick.


> 3) When I would actually disable datacow for e.g. a subvolume that
> holds VMs or DBs... what are all the implications?
> Obviously no checksumming, but what happens if I snapshot such a
> subvolume or if I send/receive it?

   After snapshotting, modifications are CoWed precisely once, and
then it reverts to nodatacow again. This means that making a snapshot
of a nodatacow object will cause it to fragment as writes are made to
it.

> I'd expect that then some kind of CoW needs to take place or does that
> simply not work?
> 
> 
> 4) Duncan mentioned that defrag (and I guess that's also for auto-
> defrag) isn't ref-link aware...
> Isn't that somehow a complete showstopper?

   It is, but the one attempt at dealing with it caused massive data
corruption, and it was turned off again. autodefrag, however, has
always been snapshot aware and snapshot safe, and would be the
recommended approach here. (Actually, it was broken in the same
incident I just described -- but fixed again when the broken patches
were reverted).

> As soon as one uses snapshot, and would defrag or auto defrag any of
> them, space usage would just explode, perhaps to the extent of ENOSPC,
> and rendering the fs effectively useless.
> 
> That sounds to me like, either I can't use ref-links, which are crucial
> not only to snapshots but every file I copy with cp --reflink auto ...
> or I can't defrag... which however will sooner or later cause quite
> some fragmentation issues on btrfs?
> 
> 
> 5) Especially keeping (4) in mind but also the other comments in from
> Duncan and Austin...
> Is auto-defrag now recommended to be generally used?

   Absolutely, yes.

   It's late for me, and this email was longer than I suspected, so
I'm going to stop here, but I'll try to pick it up again and answer
your other questions tomorrow.

   Hugo.

> Are both auto-defrag and defrag considered stable to be used? Or are
> there other implications, like when I use compression
> 
> 
> 6) Does defragmentation work with compression? Or is it just filefrag
> which can't cope with it?
> 
> Any other combinations or things with the typicaly btrfs technologies
> (cow/nowcow, compression, snapshots, subvols, compressions, defrag,
> balance) that one can do but which lead to unexpected problems (I, for
> example, wouldn't have expected that defragmentation isn't ref-link
> aware... still kinda shocked ;) )
> 
> For example, when I do a balance and change the compression, and I have
> multiple snaphots or files within one subvol that share their blocks...
> would that also lead to copies being made and the space growing
> possibly dramatically?
> 
> 
> 7) How das free-space defragmentation happen (or is there even such a
> thing)?
> For example, when I have my big qemu images, *not* using nodatacow, and
> I copy the image e.g. with qemu-img old.img new.img ... and delete the
> old then.
> Then I'd expect that the new.img is more or less not fragmented,... but
> will my free space (from the removed old.img) still be completely
> messed up sooner or later driving me into problems?
> 
> 
> 8) why does a balance not also defragment? Since everything is anyway
> copied... why not defragmenting it?
> I somehow would have hoped that a balance cleans up all kinds of
> things,... like free space issues and also fragmentation.
> 
> 
> Given all these issues,... fragmentation, situations in which space may
> grow dramatically where the end-user/admin may not necessarily expect
> it (e.g. the defrag or the balance+compression case?)... btrfs seem to
> require much more in-depth knowledge and especially care (that even
> depends on the type of data) on the end-user/admin side than the
> traditional filesystems.
> Are there for example any general recommendations what to regularly to
> do keep the fs in a clean and proper shape (and I don't count "start
> with a fresh one and 

Re: [auto-]defrag, nodatacow - general suggestions?(was: btrfs: poor performance on deleting many large files?)

[Christoph Anton Mitterer]

Hey.

I've worried before about the topics Mitch has raised.
Some questions.

1) AFAIU, the fragmentation problem exists especially for those files
that see many random writes, especially, but not limited to, big files.
Now that databases and VMs are affected by this, is probably broadly
known in the meantime (well at least by people on that list).
But I'd guess there are n other cases where such IO patterns can happen
which one simply never notices, while the btrfs continues to degrade.

So is there any general approach towards this?

And what are the actual possible consequences? Is it just that fs gets
slower (due to the fragmentation) or may I even run into other issues
to the point the space is eaten up or the fs becomes basically
unusable?

This is especially important for me, because for some VMs and even DBs
I wouldn't want to use nodatacow, because I want to have the
checksumming. (i.e. those cases where data integrity is much more
important than security)


2) Why does notdatacow imply nodatasum and can that ever be decoupled?
For me the checksumming is actually the most important part of btrfs
(not that I wouldn't like its other features as well)... so turning it
off is something I really would want to avoid.

Plus it opens questions like: When there are no checksums, how can it
(in the RAID cases) decide which block is the good one in case of
corruptions?


3) When I would actually disable datacow for e.g. a subvolume that
holds VMs or DBs... what are all the implications?
Obviously no checksumming, but what happens if I snapshot such a
subvolume or if I send/receive it?
I'd expect that then some kind of CoW needs to take place or does that
simply not work?


4) Duncan mentioned that defrag (and I guess that's also for auto-
defrag) isn't ref-link aware...
Isn't that somehow a complete showstopper?

As soon as one uses snapshot, and would defrag or auto defrag any of
them, space usage would just explode, perhaps to the extent of ENOSPC,
and rendering the fs effectively useless.

That sounds to me like, either I can't use ref-links, which are crucial
not only to snapshots but every file I copy with cp --reflink auto ...
or I can't defrag... which however will sooner or later cause quite
some fragmentation issues on btrfs?


5) Especially keeping (4) in mind but also the other comments in from
Duncan and Austin...
Is auto-defrag now recommended to be generally used?
Are both auto-defrag and defrag considered stable to be used? Or are
there other implications, like when I use compression


6) Does defragmentation work with compression? Or is it just filefrag
which can't cope with it?

Any other combinations or things with the typicaly btrfs technologies
(cow/nowcow, compression, snapshots, subvols, compressions, defrag,
balance) that one can do but which lead to unexpected problems (I, for
example, wouldn't have expected that defragmentation isn't ref-link
aware... still kinda shocked ;) )

For example, when I do a balance and change the compression, and I have
multiple snaphots or files within one subvol that share their blocks...
would that also lead to copies being made and the space growing
possibly dramatically?


7) How das free-space defragmentation happen (or is there even such a
thing)?
For example, when I have my big qemu images, *not* using nodatacow, and
I copy the image e.g. with qemu-img old.img new.img ... and delete the
old then.
Then I'd expect that the new.img is more or less not fragmented,... but
will my free space (from the removed old.img) still be completely
messed up sooner or later driving me into problems?


8) why does a balance not also defragment? Since everything is anyway
copied... why not defragmenting it?
I somehow would have hoped that a balance cleans up all kinds of
things,... like free space issues and also fragmentation.


Given all these issues,... fragmentation, situations in which space may
grow dramatically where the end-user/admin may not necessarily expect
it (e.g. the defrag or the balance+compression case?)... btrfs seem to
require much more in-depth knowledge and especially care (that even
depends on the type of data) on the end-user/admin side than the
traditional filesystems.
Are there for example any general recommendations what to regularly to
do keep the fs in a clean and proper shape (and I don't count "start
with a fresh one and copy the data over" as a valid way).


Thanks,
Chris.

> 

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