Re: [QUESTION] Upgrade xfs filesystem to reflink support?

["Darrick J. Wong" <djwong@kernel.org>]

[drop my oracle email from cc, outlook sux]

On Mon, May 09, 2022 at 10:50:20AM +0300, Amir Goldstein wrote:
> Hi Darrick and Dave,
> 
> I might have asked this back when reflink was introduced, but cannot
> find the question nor answer.
> 
> Is there any a priori NACK or exceptional challenges w.r.t implementing
> upgrade of xfs to reflink support?

No, just lack of immediate user demand + time to develop and merge code
+ time to QA the whole mess to make sure it doesn't introduce any
messes.

> We have several customers with xfs formatted pre reflink that we would
> like to consider
> upgrading.
> 
> Back in the time of reflink circa v4.9 there were few xfs features
> that could be
> upgraded, but nowadays, there are several features that could be upgraded.
> 
> If I am not mistaken, the target audience for this upgrade would be
> xfs formatted
> with xfsprogs 4.17 (defaults).
> I realize that journal size may have been smaller at that time (I need to check)
> which may be a source of additional problems,

Yes.  We've found in practice that logsize < 100MB produce serious
scalability problems and increase deadlock opportunities on such old
kernels.  The 64MB floor we just put in for xfsprogs 5.15 was a good
enough downwards estimate assuming that most people will end up on 5.19+
kernels in the (very) long run.

> but hopefully, some of your work
> to do a diet for journal credits for reflink could perhaps mitigate
> that issue(?).

That work reduces the internal transaction size but leaves the existing
minimum log size standards intact.

> Shall I take a swing at it?

It's already written:
https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfsprogs-dev.git/log/?h=upgrade-older-features

I think the upcoming nrext64 xfsprogs patches took in the first patch in
that series.

Question: Now that mkfs has a min logsize of 64MB, should we refuse
upgrades for any filesystem with logsize < 64MB?

--D

> Thanks,
> Amir.