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Re: [RFC 0/3] block/file-posix: Work around XFS bug
From: |
Stefan Hajnoczi |
Subject: |
Re: [RFC 0/3] block/file-posix: Work around XFS bug |
Date: |
Sun, 27 Oct 2019 13:35:55 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
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?
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.
Stefan
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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, no-reply, 2019/10/25
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 <=
- 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, 2019/10/28