Re: [PATCH for-4.2 v2 3/3] block/file-posix: Let post-EOF fallocate serialize

[Vladimir Sementsov-Ogievskiy]

22.08.2020 20:03, Vladimir Sementsov-Ogievskiy wrote:
> 01.11.2019 18:25, Max Reitz wrote:
> > The XFS kernel driver has a bug that may cause data corruption for qcow2
> > images as of qemu commit c8bb23cbdbe32f.  We can work around it by
> > treating post-EOF fallocates as serializing up until infinity (INT64_MAX
> > in practice).
>
> Hi! I'm doing some investigation, and here is an interesting result:
>
> Consider the following test:
>
> img=/ssd/x.qcow2; ./qemu-img create -f qcow2 $img 16G; ./qemu-img bench -c 
> 50000 -d 64 -f qcow2 -o 1k -s 64k -t none -w $img
>
>
> Bisecting results changes between 2.12 and 5.1, I found the following:
>
> 2.12: ~20s
>
> ....
>
> c8bb23cbdbe32 "qcow2: skip writing zero buffers to empty COW areas"  -> becomes 
> ~12s  [1]
>
> ....
>
>
> 292d06b925b27 "block/file-posix: Let post-EOF fallocate serialize"   -> becomes 
> ~9s  [2]
>
> ....
>
> v5.1 ~9s
>
>
> And [1] is obvious, it is the main purpose of c8bb23cbdbe32. But [2] is a 
> surprise for me.. Any ideas?
>
> ===
>
> just to check: staying at c8bb23cbdbe32 I revert c8bb23cbdbe32 and get again 
> ~19.7s. So [2] doesn't substitute [1].

Note, it's all ext4.

--
Best regards,
Vladimir