[libvirt-users] non failover equivalent to "virsh migrate --copy-storage-all"

[Eric Blake]

On 04/29/2015 09:47 AM, Andreas Buschmann wrote:
> Hello,
> 
> I have two servers where I can push VMs from one to the other by issuing
> the command
> 
>  virsh migrate --live --persistent --copy-storage-all --verbose \
>         test6 qemu+ssh://kvmhost2/system
> 
> on kvmhost1. I can get the VM back by issuing the equivalent command on
> kvmhost2:
> 
>  virsh migrate --live --persistent --copy-storage-all --verbose \
>         test6 qemu+ssh://kvmhost1/system
> 
> virsh copies the local data file /data/vm/test6.qcow2 with the
> filesystem over to the other server.
> 
> 
> Is there a way to just copy the data file over to the second server,
> without moving the VM?
> I want the equivalent of doing these two migrations is sequence, but
> without moving the VM.

Yes - 'virsh blockcopy' can be used for creating a live copy of storage
without wasting time copying memory state.  That said, blockcopy is
still waiting for a qemu 2.4 feature of persistent bitmaps before it can
work with non-transient domains (that is, in order to make the copy
operation restartable, which is necessary for a persistent domain, we
need more help from qemu, as well as patches to libvirt to use that new
qemu feature).  So for now, you will have to temporarily undefine your
domain before doing the blockcopy; you may also have to manually
coordinate quiescing the disks through the guest agent if you want your
backup to be bootable from a stable point in guest I/O (the copy you are
creating via migration is equivalent to what you get when you yank the
power cord; the disk may have unflushed pending I/O that you lose,
unless you involve the guest agent to quiesce data operations).

> 
> The goal is to get a backup copy of a running system onto a second
> system (kvmhost2).
> The copy (snapshot) from kvmhost2 can then be copied to a backup server,
> and can be used for a fast recovery point if kvmhost1 dies.

For that matter, upstream qemu has some patch series that are proposing
adding automated failover of guests, where you can get even more rapid
response time for recovering on the backup host when the primary host
fails (look for COLO on the qemu archives).  Again, it will need to land
in qemu, as well as be wired up in libvirt.

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

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