Re: Feature requests: online backup - defrag - change RAID level

Austin S. Hemmelgarn
2019-09-11T12:02:48Z

On 2019-09-10 19:32, webmaster@zedlx.com wrote:
>
> Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:
>
>
>>> Defrag may break up extents. Defrag may fuse extents. But it shouln't
>>> ever unshare extents.
>
>> Actually, spitting or merging extents will unshare them in a large
>> majority of cases.
>
> Ok, this point seems to be repeated over and over without any proof, and
> it is illogical to me.
>
> 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.

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).

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).
>
> === 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).