Re: [Qemu-devel] [PATCH v13 19/25] replay: add BH oneshot event for block layer

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Kevin Wolf [mailto:kwolf@redhat.com]
> Am 06.03.2019 um 10:37 hat Pavel Dovgalyuk geschrieben:
> > > From: Kevin Wolf [mailto:kwolf@redhat.com]
> > > Am 06.03.2019 um 10:18 hat Pavel Dovgalyuk geschrieben:
> > > > > Something like:
> > > > >
> > > > > -drive file=null-co://,if=none,id=null -device virtio-blk,drive=null
> > > >
> > > > And this drive should be destination of the copy operations, right?
> > >
> > > I don't know your exact benchmark, but this drive should be where the
> > > high I/O rates are, yes.
> >
> > Ok.
> >
> > > For getting meaningful numbers, you should have I/O only on the fast
> > > test disk (you're talking about a copy, where is source?),
> >
> > We used a qcow2 image as a source.
> 
> So the source is going to slow down the I/O and you won't actually test
> whether the possible maximum changes.
> 
> > > you should
> > > use direct I/O to get the page cache of the guest out of the way, and
> > > you should make sure that multiple requests are issued in parallel.
> >
> > Is this possible, if we have only conventional HDDs?
> 
> null-co:// doesn't access your disk at all, so if this is the only
> virtual disk that has I/O, the conventional HDD doesn't hurt. But you're
> right that you probably can't use your physical disk for high
> performance benchmarks then.
> 
> I'm going to suggest once more to use fio for storage testing. Actually,
> maybe I can find the time to do this myself on my system, too.

We've made some testing with the following fio configs:

[readtest]
blocksize=4k
filename=/dev/vda
rw=randread
direct=1
buffered=0
ioengine=libaio
iodepth=32

[writetest]
blocksize=4k
filename=/dev/vda
rw=randwrite
direct=1
buffered=0
ioengine=libaio
iodepth=32

One with read, one with write, and one with both.

master branch:
1  read : io=1024.0MB, bw=475545KB/s, iops=118886, runt=  2205msec

2  write: io=1024.0MB, bw=445444KB/s, iops=111361, runt=  2354msec

3  read : io=1024.0MB, bw=229850KB/s, iops=57462, runt=  4562msec
   write: io=1024.0MB, bw=227210KB/s, iops=56802, runt=  4615msec

rr branch:
1  read : io=1024.0MB, bw=479021KB/s, iops=119755, runt=  2189msec
2  write: io=1024.0MB, bw=440763KB/s, iops=110190, runt=  2379msec

3  read : io=1024.0MB, bw=230456KB/s, iops=57614, runt=  4550msec
   write: io=1024.0MB, bw=228548KB/s, iops=57136, runt=  4588msec

It seems that the difference can't be measured in our environment.

Pavel Dovgalyuk