[libvirt-users] virtual drive performance

Sun Jul 2 08:10:40 UTC 2017

Hi

a small update on this. I just migrated the vm from the site to my laptop
and fired it up. The exact same xml configuration (except file paths and
such) starts up and bursts with 50Mb/s to 115Mb/s in the guest. This allows
only one reasonable answer: the cpu on my laptop is somehow better suited
to emulate IO than the CPU built into the host on site. The host there is a
HP proliant microserver gen8 with xeon processor. But the processor there
is also never capped at 100% when the guest copies files.

I just ran another test by copying a 3Gb large file on the guest. What I
can observe on my computer is that the copy process is not at a constant
rate but rather starts with 90Mb/s, then drops down to 30Mb/s, goes up to
70Mb/s, drops down to 1Mb/s, goes up to 75Mb/s, drops to 1Mb/s, goes up to
55Mb/s and the pattern continues. Please note that the drive is still
configured as:

<driver name='qemu' type='qcow2' cache='none' io='threads'/>

and I would expect a constant rate that is either high or low since there
is no caching involved and the underlying hard drive is a samsung ssd evo
850. To have an idea how fast that drive is on my laptop:

$ dd if=/dev/zero of=testfile bs=1M count=1000 oflag=direct
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 2.47301 s, 424 MB/s

I can further observe that the smaller the saved chunks are the slower the
overall performance is:

dd if=/dev/zero of=testfile bs=512K count=1000 oflag=direct
1000+0 records in
1000+0 records out
524288000 bytes (524 MB, 500 MiB) copied, 1.34874 s, 389 MB/s

$ dd if=/dev/zero of=testfile bs=5K count=1000 oflag=direct
1000+0 records in
1000+0 records out
5120000 bytes (5.1 MB, 4.9 MiB) copied, 0.105109 s, 48.7 MB/s

$ dd if=/dev/zero of=testfile bs=1K count=10000 oflag=direct
10000+0 records in
10000+0 records out
10240000 bytes (10 MB, 9.8 MiB) copied, 0.668438 s, 15.3 MB/s

$ dd if=/dev/zero of=testfile bs=512 count=20000 oflag=direct
20000+0 records in
20000+0 records out
10240000 bytes (10 MB, 9.8 MiB) copied, 1.10964 s, 9.2 MB/s

Could this be a limiting factor? Does qemu/kvm do many many writes of just
a few bytes?

Ideas, anyone?

Cheers

2017-06-21 20:46 GMT+02:00 Dan <srwx4096 at gmail.com>:

> On Tue, Jun 20, 2017 at 04:24:32PM +0200, Gianluca Cecchi wrote:
> > On Tue, Jun 20, 2017 at 3:38 PM, Dominik Psenner <dpsenner at gmail.com>
> wrote:
> >
> > >
> > > to the following:
> > >
> > > <disk type='file' device='disk'>
> > >   <driver name='qemu' type='qcow2' cache='none'/>
> > >   <source file='/var/data/virtuals/machines/windows-server-2016-
> > > x64/image.qcow2'/>
> > >   <backingStore/>
> > >   <target dev='hda' bus='scsi'/>
> > >   <address type='drive' controller='0' bus='0' target='0' unit='0'/>
> > > </disk>
> > >
> > > Do you see any gotchas in this configuration that could prevent the
> > > virtualized guest to power on and boot up?
> > >
> > >
> > When I configure like this, from a linux guest point of view I get this
> > Symbios Logic SCSI Controller:
> > 00:08.0 SCSI storage controller: LSI Logic / Symbios Logic 53c895a
> >
> > But htis is true only if you add the SCSI controller too, not only the
> disk
> > definition.
> > In my case
> >
> >     <controller type='scsi' index='0'>
> >       <address type='pci' domain='0x0000' bus='0x00' slot='0x08'
> > function='0x0'/>
> >     </controller>
> >
> > Note the slot='0x08' that is reflected into the first field of lspci
> inside
> > my linux guest.
> > So between your controllers you have to add the SCSI one
> >
> > In my case (Fedora 25 with virt-manager-1.4.1-2.fc25.noarch,
> > qemu-kvm-2.7.1-6.fc25.x86_64, libvirt-2.2.1-2.fc25.x86_64) with "Disk
> bus"
> > set as SCSI in virt-manager, the xml defintiion for the guest is
> > automatically updated with the controller if not existent yet.
> > And the disk definition sections is like this:
> >
> >     <disk type='file' device='disk'>
> >       <driver name='qemu' type='qcow2'/>
> >       <source file='/var/lib/libvirt/images/slaxsmall.qcow2'/>
> >       <target dev='sda' bus='scsi'/>
> >       <boot order='1'/>
> >       <address type='drive' controller='0' bus='0' target='0' unit='0'/>
> >     </disk>
> >
> > So I think you should set dev='sda' and not 'hda' in your xml for it
> >
> I am actually very curious to know if that would make a difference. I
> don't have a such windows vm images ready to test at present.
>
> Dan
> > I don't kknow if w2016 contains the symbios logic drivers already
> > installed, so that a "simple" reboot could imply an automatic
> > reconfiguration of the guest....
> > Note also that in Windows when the hw configuration is considered heavily
> > changed, you could be asked to register again (I don't think that the IDE
> > --> SCSI should imply it...)
> >
> > Gianluca
>
> > _______________________________________________
> > libvirt-users mailing list
> > libvirt-users at redhat.com
> > https://www.redhat.com/mailman/listinfo/libvirt-users
>
>

-- 
Dominik Psenner
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