Re: [RFC 0/3] block/file-posix: Work around XFS bug

[Vladimir Sementsov-Ogievskiy]

28.10.2019 14:04, Kevin Wolf wrote:
> 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?

I'm not sure about ssd case, it may need write-zero optimization anyway.

> 
> 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.
> 

Can keeping handle_alloc_space under some config option be an option?


-- 
Best regards,
Vladimir