On 2019-09-11 13:20, webmas...@zedlx.com wrote:
Quoting "Austin S. Hemmelgarn" <ahferro...@gmail.com>:
On 2019-09-10 19:32, webmas...@zedlx.com wrote:
Quoting "Austin S. Hemmelgarn" <ahferro...@gmail.com>:
=== I CHALLENGE you and anyone else on this mailing list: ===
- Show me an exaple where splitting an extent requires unsharing,
and this split is needed to defrag.
Make it clear, write it yourself, I don't want any machine-made outputs.
Start with the above comment about all writes unsharing the region
being written to.
Now, extrapolating from there:
Assume you have two files, A and B, each consisting of 64 filesystem
blocks in single shared extent. Now assume somebody writes a few
bytes to the middle of file B, right around the boundary between
blocks 31 and 32, and that you get similar writes to file A straddling
blocks 14-15 and 47-48.
After all of that, file A will be 5 extents:
* A reflink to blocks 0-13 of the original extent.
* A single isolated extent consisting of the new blocks 14-15
* A reflink to blocks 16-46 of the original extent.
* A single isolated extent consisting of the new blocks 47-48
* A reflink to blocks 49-63 of the original extent.
And file B will be 3 extents:
* A reflink to blocks 0-30 of the original extent.
* A single isolated extent consisting of the new blocks 31-32.
* A reflink to blocks 32-63 of the original extent.
Note that there are a total of four contiguous sequences of blocks
that are common between both files:
* 0-13
* 16-30
* 32-46
* 49-63
There is no way to completely defragment either file without splitting
the original extent (which is still there, just not fully referenced
by either file) unless you rewrite the whole file to a new single
extent (which would, of course, completely unshare the whole file).
In fact, if you want to ensure that those shared regions stay
reflinked, there's no way to defragment either file without
_increasing_ the number of extents in that file (either file would
need 7 extents to properly share only those 4 regions), and even then
only one of the files could be fully defragmented.
Such a situation generally won't happen if you're just dealing with
read-only snapshots, but is not unusual when dealing with regular
files that are reflinked (which is not an uncommon situation on some
systems, as a lot of people have `cp` aliased to reflink things
whenever possible).
Well, thank you very much for writing this example. Your example is
certainly not minimal, as it seems to me that one write to the file A
and one write to file B would be sufficient to prove your point, so
there we have one extra write in the example, but that's OK.
Your example proves that I was wrong. I admit: it is impossible to
perfectly defrag one subvolume (in the way I imagined it should be done).
Why? Because, as in your example, there can be files within a SINGLE
subvolume which share their extents with each other. I didn't consider
such a case.
On the other hand, I judge this issue to be mostly irrelevant. Why?
Because most of the file sharing will be between subvolumes, not within
a subvolume.
Not necessarily. Even ignoring the case of data deduplication (which
needs to be considered if you care at all about enterprise usage, and is
part of the whole point of using a CoW filesystem), there are existing
applications that actively use reflinks, either directly or indirectly
(via things like the `copy_file_range` system call), and the number of
such applications is growing.
When a user creates a reflink to a file in the same
subvolume, he is willingly denying himself the assurance of a perfect
defrag. Because, as your example proves, if there are a few writes to
BOTH files, it gets impossible to defrag perfectly. So, if the user
creates such reflinks, it's his own whish and his own fault.
The same argument can be made about snapshots. It's an invalid argument
in both cases though because it's not always the user who's creating the
reflinks or snapshots.
Such situations will occur only in some specific circumstances:
a) when the user is reflinking manually
b) when a file is copied from one subvolume into a different file in a
different subvolume.
The situation a) is unusual in normal use of the filesystem. Even when
it occurs, it is the explicit command given by the user, so he should be
willing to accept all the consequences, even the bad ones like imperfect
defrag.
The situation b) is possible, but as far as I know copies are currently
not done that way in btrfs. There should probably be the option to
reflink-copy files fron another subvolume, that would be good.
But anyway, it doesn't matter. Because most of the sharing will be
between subvolumes, not within subvolume. So, if there is some
in-subvolume sharing, the defrag wont be 100% perfect, that a minor
point. Unimportant.
You're focusing too much on your own use case here. Not everybody uses
snapshots, and there are many people who are using reflinks very
actively within subvolumes, either for deduplication or because it saves
time and space when dealing with multiple copies of mostly identical
tress of files.
About merging extents: a defrag should merge extents ONLY when both
extents are shared by the same files (and when those extents are
neighbours in both files). In other words, defrag should always merge
without unsharing. Let's call that operation "fusing extents", so
that there are no more misunderstandings.
And I reiterate: defrag only operates on the file it's passed in. It
needs to for efficiency reasons (we had a reflink aware defrag for a
while a few years back, it got removed because performance limitations
meant it was unusable in the cases where you actually needed it).
Defrag doesn't even know that there are reflinks to the extents it's
operating on.
If the defrag doesn't know about all reflinks, that's bad in my view.
That is a bad defrag. If you had a reflink-aware defrag, and it was
slow, maybe that happened because the implementation was bad. Because, I
don't see any reason why it should be slow. So, you will have to explain
to me what was causing this performance problems.
Given this, defrag isn't willfully unsharing anything, it's just a
side-effect of how it works (since it's rewriting the block layout of
the file in-place).
The current defrag has to unshare because, as you said, because it is
unaware of the full reflink structure. If it doesn't know about all
reflinks, it has to unshare, there is no way around that.
Now factor in that _any_ write will result in unsharing the region
being written to, rounded to the nearest full filesystem block in both
directions (this is mandatory, it's a side effect of the copy-on-write
nature of BTRFS, and is why files that experience heavy internal
rewrites get fragmented very heavily and very quickly on BTRFS).
You mean: when defrag performs a write, the new data is unshared because
every write is unshared? Really?
Consider there is an extent E55 shared by two files A and B. The defrag
has to move E55 to another location. In order to do that, defrag creates
a new extent E70. It makes it belong to file A by changing the reflink
of extent E55 in file A to point to E70.
Now, to retain the original sharing structure, the defrag has to change
the reflink of extent E55 in file B to point to E70. You are telling me
this is not possible? Bullshit!
Please explain to me how this 'defrag has to unshare' story of yours
isn't an intentional attempt to mislead me.
As mentioned in the previous email, we actually did have a (mostly)
working reflink-aware defrag a few years back. It got removed because
it had serious performance issues. Note that we're not talking a few
seconds of extra time to defrag a full tree here, we're talking
double-digit _minutes_ of extra time to defrag a moderate sized (low
triple digit GB) subvolume with dozens of snapshots, _if you were lucky_
(if you weren't, you would be looking at potentially multiple _hours_ of
runtime for the defrag). The performance scaled inversely proportionate
to the number of reflinks involved and the total amount of data in the
subvolume being defragmented, and was pretty bad even in the case of
only a couple of snapshots.
Ultimately, there are a couple of issues at play here:
* Online defrag has to maintain consistency during operation. The
current implementation does this by rewriting the regions being
defragmented (which causes them to become a single new extent (most of
the time)), which avoids a whole lot of otherwise complicated logic
required to make sure things happen correctly, and also means that only
the file being operated on is impacted and only the parts being modified
need to be protected against concurrent writes. Properly handling
reflinks means that _every_ file that shares some part of an extent with
the file being operated on needs to have the reflinked regions locked
for the defrag operation, which has a huge impact on performance. Using
your example, the update to E55 in both files A and B has to happen as
part of the same commit, which can contain no other writes in that
region of the file, otherwise you run the risk of losing writes to file
B that occur while file A is being defragmented. It's not horrible when
it's just a small region in two files, but it becomes a big issue when
dealing with lots of files and/or particularly large extents (extents in
BTRFS can get into the GB range in terms of size when dealing with
really big files).
* Reflinks can reference partial extents. This means, ultimately, that
you may end up having to split extents in odd ways during defrag if you
want to preserve reflinks, and might have to split extents _elsewhere_
that are only tangentially related to the region being defragmented.
See the example in my previous email for a case like this, maintaining
the shared regions as being shared when you defragment either file to a
single extent will require splitting extents in the other file (in
either case, whichever file you don't defragment to a single extent will
end up having 7 extents if you try to force the one that's been
defragmented to be the canonical version). Once you consider that a
given extent can have multiple ranges reflinked from multiple other
locations, it gets even more complicated.
* If you choose to just not handle the above point by not letting defrag
split extents, you put a hard lower limit on the amount of fragmentation
present in a file if you want to preserve reflinks. IOW, you can't
defragment files past a certain point. If we go this way, neither of
the two files in the example from my previous email could be
defragmented any further than they already are, because doing so would
require splitting extents.
* Determining all the reflinks to a given region of a given extent is
not a cheap operation, and the information may immediately be stale
(because an operation right after you fetch the info might change
things). We could work around this by locking the extent somehow, but
doing so would be expensive because you would have to hold the lock for
the entire defrag operation.
