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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