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. 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.
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.
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.
