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

[Vladimir Sementsov-Ogievskiy]

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].

--
Best regards,
Vladimir