Re: [Qemu-devel] IO performance test on the tcm-vhost scsi

[Stefan Hajnoczi]

On Thu, Jun 14, 2012 at 05:45:22PM +0800, Cong Meng wrote:
> On Thu, 2012-06-14 at 09:30 +0100, Stefan Hajnoczi wrote:
> > On Wed, Jun 13, 2012 at 11:13 AM, mengcong <address@hidden> wrote:
> > >                    seq-read        seq-write       rand-read     
> > > rand-write
> > >                    8k     256k     8k     256k     8k   256k     8k   256k
> > > ----------------------------------------------------------------------------
> > > bare-metal          67951  69802    67064  67075    1758 29284    1969 
> > > 26360
> > > tcm-vhost-iblock    61501  66575    51775  67872    1011 22533    1851 
> > > 28216
> > > tcm-vhost-pscsi     66479  68191    50873  67547    1008 22523    1818 
> > > 28304
> > > virtio-blk          26284  66737    23373  65735    1724 28962    1805 
> > > 27774
> > > scsi-disk           36013  60289    46222  62527    1663 12992    1804 
> > > 27670
> > 
> > >
> > > unit: KB/s
> > > seq-read/write = sequential read/write
> > > rand-read/write = random read/write
> > > 8k,256k are blocksize of the IO
> > 
> > What strikes me is how virtio-blk performs significantly worse than
> > bare metal and tcm_vhost for seq-read/seq-write 8k.  The good
> > tcm_vhost results suggest that the overhead is not the virtio
> > interface itself, since tcm_vhost implements virtio-scsi.
> > 
> > To drill down on the tcm_vhost vs userspace performance gap we need
> > virtio-scsi userspace results.  QEMU needs to use the same block
> > device as the tcm-vhost-iblock benchmark.
> > 
> > Cong: Is it possible to collect the virtio-scsi userspace results
> > using the same block device as tcm-vhost-iblock and -drive
> > format=raw,aio=native,cache=none?
> > 
> 
> virtio-scsi-raw     43065  69729    52052  67378    1757 29419    2024 28135
> 
> qemu ....\
> -drive file=/dev/sdb,format=raw,if=none,id=sdb,cache=none,aio=native \
> -device virtio-scsi-pci,id=mcbus \
> -device scsi-disk,drive=sdb
> 
> there is only one scsi HBA. 
> /dev/sdb is the disk on which all tests have been done. 
> 
> Is this what you want?

Perfect, thanks.  virtio-scsi userspace is much better than virtio-blk
here.  That's unexpected since they both use the QEMU block layer.  If
anything, I would have expected virtio-blk to be faster!

I wonder if the request patterns being sent through virtio-blk and
virtio-scsi are different.  Asias discovered that the guest I/O
scheduler and request merging makes a big difference between QEMU and
native KVM tool performance.  It could be the same thing here which
causes virtio-blk and virtio-scsi userspace to produce quite different
results.

The second question is why is tcm_vhost faster than virtio-scsi
userspace.

Stefan