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

[Vladimir Sementsov-Ogievskiy]

26.08.2020 11:23, Vladimir Sementsov-Ogievskiy wrote:
> 22.08.2020 20:04, Vladimir Sementsov-Ogievskiy wrote:
> > 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.
>
>
> Let's go further:
>
> If I add -n, to use aio native:
> ./qemu-img create -f qcow2 $img 16G; ./qemu-img bench -c 50000 -d 64 -f qcow2 
> -o 1k -s 64k -t none -n -w $img;
>
> Then this patch doesn't have such effect..
>
> But if consider the test without an offset on hdd, the effect is very 
> significant:
>
> ./qemu-img create -f qcow2 $img 16G; ./qemu-img bench -c 45000 -d 64 -f qcow2  
> -s 16k -t none -n -w $img;
>
> v2.12 ~10.4s
> c8bb23cbdbe32^ ~10.6s
> c8bb23cbdbe32 qcow2: skip writing zero buffers to empty COW areas: ~16.7s : 
> degradation!!!
> 292d06b925b27^ ~16.4s
> 292d06b925 block/file-posix: Let post-EOF fallocate serialize: ~7.6s: great 
> improvement !

Hmm, a bit more accurate results (use separate partition on non-root hard 
drive). I post just numbers I get from iterations, not calculating the average.

v2.12.0 9.227 8.999 9.059 8.793
c8bb23cbdbe32^ 9.264 8.996 8.969
c8bb23cbdbe32 20.694 16.050 15.898 15.918 16.136
292d06b925b27^ 15.509 15.343 16.549
292d06b925 8.368 8.529 8.342 7.503 8.361 8.353 8.416 13.043 12.594 7.898 7.907 
11.914

292d06b925 + don't-handle-alloc 18.730 9.321 9.016 9.037 18.286 18.268

(don't handle alloc is don't call qcow2_handle_alloc function which is actual 
revert of c8bb23cbdbe32)

very interesting..

292d06b925^ + don't-handle-alloc 9.200 9.285 9.297 9.296 9.194 9.332 9.155

--
Best regards,
Vladimir