[libvirt-users] virsh blockcommit fails regularily (was: virtual drive performance)
Dominik Psenner
dpsenner at gmail.com
Mon Aug 14 06:42:24 UTC 2017
Hi,
a small update on this. We have migrated the virtualized host to use the
virtio drivers and now the drive performance is improved so that we can see
a constant transfer rate. Before it used to be the same rate but regularly
dropped to a few bytes/sec for a few seconds and then was fast again.
However we still observe that the following fails regularily:
$ virsh snapshot-create-as --domain domain --name backup --no-metadata
--atomic --disk-only --diskspec hda,snapshot=external
$ virsh blockcommit domain hda --active --pivot
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.
Then running the following in the morning succeeds and successfully pivotes
the snapshot into the base image while the vm is live:
$ virsh blockjob domain hda --abort
$ virsh blockcommit domain hda --active --pivot
Successfully pivoted
We run the backup process every day once and it failed on the following
days:
2017-07-07
2017-07-20
2017-07-27
2017-08-12
2017-08-14
Looking at this it roughly happens once a week and the guest from then on
writes into the snapshot backlog. That snapshot backlog file grows about
8gb every day and thus the issue always needs immediate attention.
Any ideas what could cause this issue? Is this a bug (race condition) of
`virsh blockcommit` that sometimes fails because it is invoked at the wrong
time?
Cheers,
Dominik
2017-07-07 9:21 GMT+02:00 Dominik Psenner <dpsenner at gmail.com>:
> Of course the cronjob fails when trying to virsh blockcommit and not when
> creating the snapshot, sorry for the noise.
>
> 2017-07-07 9:15 GMT+02:00 Dominik Psenner <dpsenner at gmail.com>:
>
>> Hi,
>>
>> different day, same issue.. cronjob runs and fails:
>>
>> $ virsh snapshot-create-as --domain domain --name backup --no-metadata
>> --atomic --disk-only --diskspec hda,snapshot=external
>> 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.
>>
>> Then running the following in the morning succeeds and successfully
>> pivotes the snapshot into the base image while the vm is live:
>>
>> $ virsh blockjob domain hda --abort
>> $ virsh blockcommit domain hda --active --pivot
>> Successfully pivoted
>>
>> This need of manual interventions is becoming a tiring job..
>>
>> I someone else seeing the same issue or has an idea what the cause could
>> be?
>> Can I trust the output and is the base image really up to the latest
>> state?
>>
>> Cheers
>>
>> 2017-07-02 10:30 GMT+02:00 Dominik Psenner <dpsenner at gmail.com>:
>>
>>> Just a little catch-up. This time I was able to resolve the issue by
>>> doing:
>>>
>>> virsh blockjob domain hda --abort
>>> virsh blockcommit domain hda --active --pivot
>>>
>>> Last time I had to shut down the virtual machine and do this while being
>>> offline.
>>>
>>> Thanks Wang for your valuable input. As far as the memory goes, there's
>>> plenty of head room:
>>>
>>> $ free -h
>>> total used free shared buff/cache
>>> available
>>> Mem: 7.8G 1.8G 407M 9.7M
>>> 5.5G 5.5G
>>> Swap: 8.0G 619M 7.4G
>>>
>>> 2017-07-02 10:26 GMT+02:00 王李明 <wanglm at certusnet.com.cn>:
>>>
>>>> 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 at 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 at 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 at gmail.com>:
>>>>
>>>> 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
>>>>
>>>>
>>>>
>>>>
>>>> --
>>>>
>>>> Dominik Psenner
>>>>
>>>
>>>
>>>
>>> --
>>> Dominik Psenner
>>>
>>
>>
>>
>> --
>> Dominik Psenner
>>
>
>
>
> --
> Dominik Psenner
>
--
Dominik Psenner
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