Re: Problems with c8bb23cbdbe3 on ppc64le

[Anton Nefedov]

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?)

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).

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.

/Anton