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Re: [RFC 0/3] block/file-posix: Work around XFS bug
From: |
Kevin Wolf |
Subject: |
Re: [RFC 0/3] block/file-posix: Work around XFS bug |
Date: |
Mon, 28 Oct 2019 12:04:08 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
Am 27.10.2019 um 13:35 hat Stefan Hajnoczi geschrieben:
> On Fri, Oct 25, 2019 at 11:58:46AM +0200, Max Reitz wrote:
> > As for how we can address the issue, I see three ways:
> > (1) The one presented in this series: On XFS with aio=native, we extend
> > tracked requests for post-EOF fallocate() calls (i.e., write-zero
> > operations) to reach until infinity (INT64_MAX in practice), mark
> > them serializing and wait for other conflicting requests.
> >
> > Advantages:
> > + Limits the impact to very specific cases
> > (And that means it wouldn’t hurt too much to keep this workaround
> > even when the XFS driver has been fixed)
> > + Works around the bug where it happens, namely in file-posix
> >
> > Disadvantages:
> > - A bit complex
> > - A bit of a layering violation (should file-posix have access to
> > tracked requests?)
>
> Your patch series is reasonable. I don't think it's too bad.
>
> The main question is how to detect the XFS fix once it ships. XFS
> already has a ton of ioctls, so maybe they don't mind adding a
> feature/quirk bit map ioctl for publishing information about bug fixes
> to userspace. I didn't see another obvious way of doing it, maybe a
> mount option that the kernel automatically sets and that gets reported
> to userspace?
I think the CC list is too short for this question. We should involve
the XFS people here.
> If we imagine that XFS will not provide a mechanism to detect the
> presence of the fix, then could we ask QEMU package maintainers to
> ./configure --disable-xfs-fallocate-beyond-eof-workaround at some point
> in the future when their distro has been shipping a fixed kernel for a
> while? It's ugly because it doesn't work if the user installs an older
> custom-built kernel on the host. But at least it will cover 98% of
> users...
>
> > (3) Drop handle_alloc_space(), i.e. revert c8bb23cbdbe32f.
> > To my knowledge I’m the only one who has provided any benchmarks for
> > this commit, and even then I was a bit skeptical because it performs
> > well in some cases and bad in others. I concluded that it’s
> > probably worth it because the “some cases” are more likely to occur.
> >
> > Now we have this problem of corruption here (granted due to a bug in
> > the XFS driver), and another report of massively degraded
> > performance on ppc64
> > (https://bugzilla.redhat.com/show_bug.cgi?id=1745823 – sorry, a
> > private BZ; I hate that :-/ The report is about 40 % worse
> > performance for an in-guest fio write benchmark.)
> >
> > So I have to ask the question about what the justification for
> > keeping c8bb23cbdbe32f is. How much does performance increase with
> > it actually? (On non-(ppc64+XFS) machines, obviously)
> >
> > Advantages:
> > + Trivial
> > + No layering violations
> > + We wouldn’t need to keep track of whether the kernel bug has been
> > fixed or not
> > + Fixes the ppc64+XFS performance problem
> >
> > Disadvantages:
> > - Reverts cluster allocation performance to pre-c8bb23cbdbe32f
> > levels, whatever that means
>
> My favorite because it is clean and simple, but Vladimir has a valid
> use-case for requiring this performance optimization so reverting isn't
> an option.
Vladimir also said that qcow2 subclusters would probably also solve his
problem, so maybe reverting and applying the subcluster patches instead
is a possible solution, too?
We already have some cases where the existing handle_alloc_space()
causes performance to actually become worse, and serialising requests as
a workaround isn't going to make performance any better. So even on
these grounds, keeping commit c8bb23cbdbe32f is questionable.
Kevin
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- Re: [RFC 0/3] block/file-posix: Work around XFS bug, (continued)
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Nir Soffer, 2019/10/26
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Stefan Hajnoczi, 2019/10/27
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/28
Re: [RFC 0/3] block/file-posix: Work around XFS bug,
Kevin Wolf <=
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/28
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Vladimir Sementsov-Ogievskiy, 2019/10/29
- Re: [RFC 0/3] block/file-posix: Work around XFS bug, Max Reitz, 2019/10/29