Re: [Qemu-discuss] Live VM backup

[Fam Zheng]

On Wed, 08/30 12:07, Vincenzo Romano wrote:
> 2017-08-30 11:58 GMT+02:00 Fam Zheng <address@hidden>:
> > On Wed, 08/30 11:45, Vincenzo Romano wrote:
> >> 2017-08-30 11:40 GMT+02:00 Fam Zheng <address@hidden>:
> >> > On Wed, 08/30 11:16, Vincenzo Romano wrote:
> >> >> 2017-08-30 10:53 GMT+02:00 Fam Zheng <address@hidden>:
> >> >> > On Wed, 08/30 08:17, Vincenzo Romano wrote:
> >> >> >> Il 30 ago 2017 05:19, "Fam Zheng" <address@hidden> ha scritto:
> >> >> >>
> >> >> >> On Tue, 08/29 18:32, Vincenzo Romano wrote:
> >> >> >> > I'd like to move away from "another virtualization solution" to 
> >> >> >> > QEmu.
> >> >> >> > I've more or less all the needed bits in my hands in order to 
> >> >> >> > define,
> >> >> >> > run and manage QEmu VMs.
> >> >> >> > What I am missing is the knowledge of how to backup a live VM.
> >> >> >> > Of course, if I shutdown the VM, I can simply copy its files and 
> >> >> >> > the
> >> >> >> > startup script on another QEmu host.
> >> >> >> > But my main objective is to do the backup while the VM is running.
> >> >> >> > I think I could use the snapshot feature, but have no idea on how 
> >> >> >> > to
> >> >> >> proceed.
> >> >> >> > Any hint?
> >> >> >>
> >> >> >> In short, just do what libvirt would do: use QMP and send 
> >> >> >> 'drive-backup'
> >> >> >> command.
> >> >> >>
> >> >> >> This is a very good howto:
> >> >> >>
> >> >> >> https://github.com/qemu/qemu/blob/master/docs/interop/live-b
> >> >> >> lock-operations.rst
> >> >> >>
> >> >> >> Fam
> >> >> >>
> >> >> >>
> >> >> >> Thanks.
> >> >> >> I presume there's a 'drive-restore', though I need to look better in 
> >> >> >> the
> >> >> >> docs.
> >> >> >>
> >> >> >> Is there any side effect in case the CPU where the VM will be 
> >> >> >> restored is
> >> >> >> not exactly the same as the one where the VM got backed up?
> >> >> >
> >> >> > No, there isn't a 'drive-restore'. 'drive-backup' only takes care of 
> >> >> > the disks,
> >> >> > so the CPU, ram and device states aren't saved.
> >> >> >
> >> >> > If you boot from the backup image, CPU and ram are restarted from 
> >> >> > powered off
> >> >> > state, so what happens is effectively like a power loss at the point 
> >> >> > of
> >> >> > 'drive-backup': anything not saved to disk are lost.
> >> >> >
> >> >> > Fam
> >> >>
> >> >> I understand.
> >> >>
> >> >> So, if I haven't miunderstood, at the moment QEmu-KVM doesn't
> >> >> implement  "VM snapshots".
> >> >
> >> > It does. There are "savevm" and "migrate" (or "migrate -b") that can 
> >> > save the
> >> > CPU/ram etc:
> >> >
> >> > https://www.linux-kvm.org/page/Migration#savevm.2Floadvm_to_an_external_state_file_.28using_pseudo-migration.29
> >> >
> >> > Fam
> >>
> >> Thanks again, Fam.
> >>
> >> So I should issue both QMP commands, in theory, with the VM in a
> >> frozen state. Right?
> >
> > I think with "savevm", your VM is automatically paused when the command is 
> > being
> > handled. The "migrate -b 'exec: ...'" way is an alternative and doesn't 
> > need the
> > VM to stop, which should be better.
> >
> > Fam
> 
> Thanks.
> What if the two hosts have "slightly different" CPU capabilities?
> I have a main host with a  "Xeon E5-2620 v3" while the backup one is a
> "Xeon E5-2407".
> Apart of the performances, the former is lacking some instruction set
> extensions:
> SSE2, MMX, SSE3, SSE, SSE4.1, SSE4.2, Supplemental SSE3 and AES
> despite being much newer than the latter which "only" lacks AVX 2.0.

You can control CPU flags from the command line, and for the same reason, the
machine types and PCI addrs, etc.

> 
> In this case the current virtualization technology is showing its 
> deficiencies.
> For a number of reasons I am also testing this type of "corner cases",
> when a backup cannot be restored on exactly the same hardware it comes
> from.

At this point you are already reinventing libvirt, so the answer is either use
it or at least dig into how it manages QEMU :). It would be useful to inspect
the QEMU command line it generates and the QMP commands it issues for certain
management tasks, such as backup, hotplug and migration, etc.

Fam