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

webmaster
2019-09-11T17:21:01Z

Quoting "Austin S. Hemmelgarn" <ahferroin7@gmail.com>:

> On 2019-09-10 19:32, webmaster@zedlx.com wrote:
>>
>> Quoting "Austin S. Hemmelgarn" <ahferroin7@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.