[libvirt] qemu: managedsave vs. save

[Richard Weinberger]

On Fri, May 2, 2014 at 3:43 PM, Laine Stump <laine at laine.org> wrote:
> On 05/02/2014 03:38 PM, Richard Weinberger wrote:
>> On Fri, May 2, 2014 at 2:16 PM, Daniel P. Berrange <berrange at redhat.com> wrote:
>>> On Fri, May 02, 2014 at 02:08:28PM +0200, Richard Weinberger wrote:
>>>> Hi!
>>>>
>>>> My KVM hosts share the same filesystem and I'm facing an issue using
>>>> managedsave.
>>>> If I save vmX using managedsave on hostA and restore it later using
>>>> "virsh restore" in hostB
>>>> the qemu process consumes 100% CPU and makes no progress.
>>>> On the other hand, if I save vmX using save the restore works fine on hostB.
>>> FWIW, you use 'managedsave' then you shouldn't use 'restore' - you
>>> should just 'start' the guest as normal and libvirt will automagically
>>> use the managed save image.
>>>
>>> Of course this doesn't explain the problem you see - the result of
>>> managedsave should be identical to the result of save. They use the
>>> same QEMU code internally, the only difference being that in one
>>> case you decide the filename and in the other case libvirt decides
>>> the filename
>> BTW: On a different system with libvirtd 1.2.3 restore works fine
>> from managedsave'd domains.
>>
>> Looks like I have to upgrade my libvirt. :-\
>
> But you still should not use a combination of managedsave + [manually
> moving the config & save file to other system] + restore to migrate a
> guest from one host to another. This is misuse of managedsave, and extra
> work that you shouldn't have to do. (managedsave is really intended to
> be used to temporarily pause a guest so that, e.g., the host can be
> rebooted, after which the guest can be restarted *on the same host* by
> using virsh start; this is used by the libvirt-guests service. Although
> starting this saved image on a different host may currently work, there
> is no guarantee that it will continue to work in the future).

Then please clearly state this in the man page and create .save images
which is unable to be resumed by "restore" to prevent further misuse. :)

> Instead look into the virsh migrate command. It will do all the dirty
> work for you, including removing the guest definition from the source
> host and defining it on the destination host if you want, and it
> minimizes guest downtime to "something very short". It can also migrate
> the storage if you need it to (although this increases the migration
> time considerably).

I cannot use migrate because I'm using DRBD.
Takeover from hostA to hostB works as follows:
1. save all VMs do disk
2. unmount the fs on hostA
3. make hostA secondary in the DBRD cluster
4. make hostB primary
5. mount the fs on hostB
6. restore saved VMs on hostB

Looks like I have to update /etc/init.d/libvirt-guests to use a plain "save"
instead of "managedsave".

-- 
Thanks,
//richard