[Users] Online snapshot

[Shu Ming]

After looking into a qemu a bit,  I believe the snapshot operation on 
the disk in oVirt is not a live snapshot, because the guest VM will be 
paused by qemu when the snapshot is undergoing.    That is what qemu 
monitor command "snapshot_blkdev" does and VDSM depends on that command 
in low level.    Most likely,  the pausing time is non-trival for a 
normal snapshot.

On 2012-6-29 0:37, Itamar Heim wrote:
> On 06/28/2012 09:20 AM, Eric Blake wrote:
>> On 06/28/2012 07:07 AM, Dave Allan wrote:
>>> On Thu, Jun 28, 2012 at 06:34:25AM -0400, Itamar Heim wrote:
>>>> On 06/28/2012 04:40 AM, Winfried de Heiden wrote:
>>>>> Hi all,
>>>>>
>>>>> It seems live snapshot is available in the Fedora 17 qemu-kvm. On my
>>>>> laptop with kvm, virt-manager, libvirt, virsh etc. I created a 
>>>>> virtual
>>>>> machine and tried  to make a live snapshot:
>>>>>
>>
>>>>>
>>>>> virsh # snapshot-create RHEL55_NTP
>>>>> Domain snapshot 1340871802 created
>>
>> That's a system-checkpoint snapshot, because you didn't use --disk-only.
>>   When you say 'live snapshot' were you intending the fast operation of
>> just a disk snapshot (via the 'transaction' or 'blockdev-snapshot-sync'
>> monitor command) or the slow operation of a checkpoint (via the 'savevm'
>> monitor command)?
>>
>>>>> [root at kvm ~]# rpm -qf `which qemu-kvm`
>>>>> qemu-system-x86-1.0-17.fc17.x86_64
>>>>>
>>>>> [wdh at laptop-winfried ~]$ rpm -qf `which qemu-kvm`
>>>>> qemu-system-x86-1.0-17.fc17.x86_64
>>
>> This qemu does not support the 'transaction' monitor command, so libvirt
>> has to fall back to the non-atomic 'blockdev-snapshot-sync', or else
>> give up because of missing functionality.
>>
>>>>
>>>> dave - who can help us elaborate on this libvirt error:
>>>> reuse is not supported with
>>>> this QEMU binary
>>
>>
>>>>>>
>>>>>> this is the relevant error coming from libvirt.
>>>>>> maybe fedora 17 qemu-kvm does not have live snapshot support?
>>>>>>
>>>>>> Thread-306::ERROR::2012-06-27
>>>>>> 12:46:40,178::libvirtvm::1782::vm.Vm::(snapshot)
>>>>>> vmId=`ef978066-09c8-40aa-8e0b-4accfacc15bd`::Unable to take snapshot
>>>>>> Traceback (most recent call last):
>>>>>>    File "/usr/share/vdsm/libvirtvm.py", line 1780, in snapshot
>>>>>>      libvirt.VIR_DOMAIN_SNAPSHOT_CREATE_REUSE_EXT))
>>
>> OK, so this is a case of VDSM driving a different use of the API than
>> what you did with your virsh command line.  VDSM is attempting to do the
>> equivalent of 'virsh snapshot-create --disk-only --reuse-ext', but
>> because your qemu doesn't support the 'transaction' command, libvirt
>> can't make qemu do what VDSM wants.  Basically, the
>> 'blockdev-snapshot-sync' command in qemu 1.0 lacks the ability to reuse
>> qcow2 metadata already present in the file pre-created by VDSM; this
>> wasn't added until the 'transaction' command of qemu 1.1.
>>
>
> ok, i guess the question is then when qemu 1.1 is expected in fedora
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-- 
Shu Ming <shuming at linux.vnet.ibm.com>
IBM China Systems and Technology Laboratory