Re: Problems with c8bb23cbdbe3 on ppc64le

[Max Reitz]

On 10.10.19 18:15, Anton Nefedov wrote:
> On 10/10/2019 6:17 PM, Max Reitz wrote:
>> Hi everyone,
>>
>> (CCs just based on tags in the commit in question)
>>
>> I have two bug reports which claim problems of qcow2 on XFS on ppc64le
>> machines since qemu 4.1.0.  One of those is about bad performance
>> (sorry, is isn’t public :-/), the other about data corruption
>> (https://bugzilla.redhat.com/show_bug.cgi?id=1751934).
>>
>> It looks like in both cases reverting c8bb23cbdbe3 solves the problem
>> (which optimized COW of unallocated areas).
>>
>> I think I’ve looked at every angle but can‘t find what could be wrong
>> with it.  Do any of you have any idea? :-/
>>
> 
> hi,
> 
> oh, that patch strikes again..
> 
> I don't quite follow, was this bug confirmed to happen on x86? Comment 8
> (https://bugzilla.redhat.com/show_bug.cgi?id=1751934#c8) mentioned that
> (or was that mixed up with the old xfsctl bug?)

I think that was mixed up with the xfsctl bug, yes.

> Regardless of the platform, does it reproduce? That's comforting
> already; worst case we can trace each and every request then (unless it
> will stop to reproduce this way).

I haven’t been able to reproduce it yet (wrestling with the test system
and getting ppc64 machines provisioned), but as far as I know it
reproduces reliably on ppc64, but only there.

> Also, perhaps it's worth to try to replace fallocate with write(0)?
> Either in qcow2 (in the patch, bdrv_co_pwrite_zeroes -> bdrv_co_pwritev)
> or in the file driver. It might hint whether it's misbehaving fallocate
> (in qemu or in kernel) or something else.

Good idea, that should at least tell us something about the corruption.

Max

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