[Users] Online snapshot

[Eric Blake]

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.

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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