Re: [libvirt-users] virtual drive performance

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(a)gmail.com&gt;: > 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(a)gmail.com&gt; > 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(a)redhat.com > > https://www.redhat.com/mailman/listinfo/libvirt-users > > -- Dominik Psenner