mybe this is because you physic host memory is small

then this will Causing instability of the virtual machine

But I'm just guessing

You can try to increase your memory

Wang Liming

发件人: libvirt-users-bounces@redhat.com [mailto:libvirt-users-bounces@redhat.com] 代表 Dominik Psenner
发送时间: 2017年7月2日 16:22
收件人: libvirt-users@redhat.com
主题: Re: [libvirt-users] virtual drive performance

Hi again,

just today an issue I've thought to be resolved popped up again. We backup the machine by doing:

virsh snapshot-create-as --domain domain --name backup --no-metadata --atomic --disk-only --diskspec hda,snapshot=external

# backup hda.qcow2

virsh blockcommit domain hda --active --pivot

Every now and then this process fails with the following error message:

error: failed to pivot job for disk hda
error: block copy still active: disk 'hda' not ready for pivot yet
Could not merge changes for disk hda of domain. VM may be in invalid state.

I expect live backups are a great asset and should work. Is this a bug that may relates also to the virtual drive performance issues we observe?

Cheers

2017-07-02 10:10 GMT+02:00 Dominik Psenner <dpsenner@gmail.com>:

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@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@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

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Dominik Psenner