Quoting "Austin S. Hemmelgarn" <ahferro...@gmail.com>:
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>:
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.
You cannot ever make the worst program, because an even worse program
can be made by slowing down the original by a factor of 2.
So, you had a badly implemented defrag. At least you got some
experience. Let's see what went wrong.
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.
Nah. I think there is a workaround. You can first (atomically) update
A, then whatever, then you can update B later. I know, your yelling
"what if E55 gets updated in B". Doesn't matter. The defrag continues
later by searching for reflink to E55 in B. Then it checks the data
contained in E55. If the data matches the E70, then it can safely
update the reflink in B. Or the defrag can just verify that neither
E55 nor E70 have been written to in the meantime. That means they
still have the same data.
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).
You must just split large extents in a smart way. So, in the
beginning, the defrag can split large extents (2GB) into smaller ones
(32MB) to facilitate more responsive and easier defrag.
If you have lots of files, update them one-by one. It is possible. Or
you can update in big batches. Whatever is faster.
The point is that the defrag can keep a buffer of a "pending
operations". Pending operations are those that should be performed in
order to keep the original sharing structure. If the defrag gets
interrupted, then files in "pending operations" will be unshared. But
this should really be some important and urgent interrupt, as the
"pending operations" buffer needs at most a second or two to complete
its operations.
* 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.
I think that this problem can be solved, and that it can be solved
perfectly (the result is a perfectly-defragmented file). But, if it is
so hard to do, just skip those problematic extents in initial version
of defrag.
Ultimately, in the super-duper defrag, those partially-referenced
extents should be split up by defrag.
* 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.
Oh, you're reading my thoughts. That's good.
Initial implementation of defrag might be not-so-perfect. It would
still be better than the current defrag.
This is not a one-way street. Handling of partially-used extents can
be improved in later versions.
* 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.
No. DO NOT LOCK TO RETRIEVE REFLINKS.
Instead, you have to create a hook in every function that updates the
reflink structure or extents (for exaple, write-to-file operation).
So, when a reflink gets changed, the defrag is immediately notified
about this. That way the defrag can keep its data about reflinks
in-sync with the filesystem.
Also note, this defrag should run as a part of the kernel, not in
userspace. Defrag-from-userspace is a nightmare. Defrag has to
serialize its operations properly, and it must have knowledge of all
other operations in progress. So, it can only operate efficiently as
part of the kernel.
