libvirt users - Feb 2015 - Re: [libvirt] vm live storage migration with snapshots

Hi Eric,

Please see the blew:

On Wed, Feb 11, 2015 at 3:12 PM, Eric Blake <eblake@redhat.com> wrote:
> On 02/11/2015 02:07 PM, Edward Young wrote:
> >>> What if this vm has a number of disk-only external snapshots?
In the
> >>> current version, how can live migrate this vm?
> >>
> >> Are the snapshots based on shared storage, or local-only storage?
> >>
> >
> > Yes, I'm talking about the local-only storage.
>
> Okay, glad I guessed as much, then.
>
> >
> > Yes, I agree with you. In this case, we need to migrate the entire
disk
> > state. In this case, there is no snapshot involved. we just perform
the
> > regular migration with 'virsh migrate....'. Is this correct?
>
> Using 'virsh migrate --copy-storage-all' would indeed migrate the
entire
> disk, if you can't supply shared storage.
>
> >>> Or is it possible to iteratively transfer all the snapshots to
the
> >>> destination and later live migrate only the latest new data?
> >>>
> >>
> >> Yes, that works too, and is probably faster, especially if you
have
> >> out-of-band means for sharing read-only state between source and
> >> destination.
> >>
> >
> > My question is here. If we do not have any shared storage resource
> between
> > source and destination (eg. long distance VM migration), how can we
> migrate
> > the latest new data to the destination? we can copy the base, mid to
> > destination manually, then how can we migrate the active snapshot( new
> data
> > goes in)? I learned that drive_mirror in qemu is built to finish this,
> but
> > do not understand clearly. Could you elaborate for me, or provide an
> > example?
>
> Use 'virsh migrate --copy-storage-inc' to migrate only the
incremental
> changes, which assumes that the destination can already see the same
> read-only backing data that the source sees.

I perform a simple test, but failed.

In the source, I create:   base <- mid <- active (2 snapshots, the active
one is the current one)
In order to migrate this vm to the destination, I manually copy both base
and mid to the destination, and put them in the sam e directory
/var/lib/libvirt/images/
Then I call the following commands:
'sudo virsh migrate --live --unsafe --verbose --copy-storage-inc vm1
qemu+ssh://192.168.1.3/system tcp://192.168.1.3'

The error I got is:
error: Requested operation is not valid: cannot migrate domain with 2
snapshots.

I do not know the reason for this.

> In fact, modern
> libvirt/qemu does this for you by setting up an NBD server on the
> destination, doing a data mirror from the source into the destination
> (so that you DO have shared storage, at least for the duration of the
> migration), then doing the live migration, then tearing down the NBD
> mirroring link.
>
Sounds interesting! Can you provide me an example or point me to a link
about how to use this function?

Sorry to take you so much time to answer my questions. I appreciate it
deeply. After I solve this issue, I will write a blog and share these
knowledge with other people.

Best,
Ed

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

[dropping multiple lists; let's just use libvirt-users]

On 02/11/2015 02:45 PM, Edward Young wrote:
> I perform a simple test, but failed.
> 
> In the source, I create:   base <- mid <- active (2 snapshots, the
active
> one is the current one)
> In order to migrate this vm to the destination, I manually copy both base
> and mid to the destination, and put them in the sam e directory
> /var/lib/libvirt/images/
> Then I call the following commands:
> 'sudo virsh migrate --live --unsafe --verbose --copy-storage-inc vm1
> qemu+ssh://192.168.1.3/system tcp://192.168.1.3'
> 
> The error I got is:
> error: Requested operation is not valid: cannot migrate domain with 2
> snapshots.
> 
> I do not know the reason for this.
That's because libvirt is not (yet) able to migrate snapshots in a way
that the destination can revert to them.  It is possible to work around
this in a couple of ways:

1. Manually dump all the snapshot XML configurations on the source side
(virsh snapshot-list, then virsh snapshot-dumpxml in a loop), then
delete the snapshot tracking (virsh snapshot-delete --metadata in a
loop), then do the migration, then manually recreate the snapshot
configuration on the destination (virsh snapshot-create --redefine in a
loop for each of the xml files you saved earlier).

If you don't plan to revert to snapshots on the destination, then you
can skip the snapshot-dumpxml loop, as well as the snapshot-create
--redefine loop.  But if that's the case, then you can take a shortcut:

2. Don't track external snapshot data in libvirt in the first place
(virsh snapshot-create --no-metadata when taking external snapshots).
Instead, the backing chain is sufficient to know that external snapshot
data was saved.
> 
> 
>> In fact, modern
>> libvirt/qemu does this for you by setting up an NBD server on the
>> destination, doing a data mirror from the source into the destination
>> (so that you DO have shared storage, at least for the duration of the
>> migration), then doing the live migration, then tearing down the NBD
>> mirroring link.
>>
> 
> Sounds interesting! Can you provide me an example or point me to a link
> about how to use this function?
It's all done under the hood as part of the implementation of 'virsh
migrate --copy-source-*', if done as part of migration.  But if you want
to manually recreate the steps, the source code in
src/qemu/qemu_migration.c shows the QMP commands used to set up the NBD
server on the destination, then the mirroring is done with equivalent of
the 'virsh blockcopy' command pointing to the NBD server as the
destination.
> 
> Sorry to take you so much time to answer my questions. I appreciate it
> deeply. After I solve this issue, I will write a blog and share these
> knowledge with other people.
The libvirt-users list archives has a lot of questions and posts from
other users on the same topic; it may also help if you peruse the archives.

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

On Wed, Feb 11, 2015 at 5:09 PM, Eric Blake <eblake@redhat.com> wrote:
> [dropping multiple lists; let's just use libvirt-users]
>
> On 02/11/2015 02:45 PM, Edward Young wrote:
>
> > I perform a simple test, but failed.
> >
> > In the source, I create:   base <- mid <- active (2 snapshots,
the active
> > one is the current one)
> > In order to migrate this vm to the destination, I manually copy both
base
> > and mid to the destination, and put them in the sam e directory
> > /var/lib/libvirt/images/
> > Then I call the following commands:
> > 'sudo virsh migrate --live --unsafe --verbose --copy-storage-inc
vm1
> > qemu+ssh://192.168.1.3/system tcp://192.168.1.3'
> >
> > The error I got is:
> > error: Requested operation is not valid: cannot migrate domain with 2
> > snapshots.
> >
> > I do not know the reason for this.
>
> That's because libvirt is not (yet) able to migrate snapshots in a way
> that the destination can revert to them.  It is possible to work around
> this in a couple of ways:
>
> 1. Manually dump all the snapshot XML configurations on the source side
> (virsh snapshot-list, then virsh snapshot-dumpxml in a loop), then
> delete the snapshot tracking (virsh snapshot-delete --metadata in a
> loop), then do the migration, then manually recreate the snapshot
> configuration on the destination (virsh snapshot-create --redefine in a
> loop for each of the xml files you saved earlier).
>
> If you don't plan to revert to snapshots on the destination, then you
> can skip the snapshot-dumpxml loop, as well as the snapshot-create
> --redefine loop.  But if that's the case, then you can take a shortcut:
>
> 2. Don't track external snapshot data in libvirt in the first place
> (virsh snapshot-create --no-metadata when taking external snapshots).
> Instead, the backing chain is sufficient to know that external snapshot
> data was saved.
>
It works for me. Thanks!
>
> >
> >
> >> In fact, modern
> >> libvirt/qemu does this for you by setting up an NBD server on the
> >> destination, doing a data mirror from the source into the
destination
> >> (so that you DO have shared storage, at least for the duration of
the
> >> migration), then doing the live migration, then tearing down the
NBD
> >> mirroring link.
> >>
> >
> > Sounds interesting! Can you provide me an example or point me to a
link
> > about how to use this function?
>
> It's all done under the hood as part of the implementation of
'virsh
> migrate --copy-source-*', if done as part of migration.  But if you
want
> to manually recreate the steps, the source code in
> src/qemu/qemu_migration.c shows the QMP commands used to set up the NBD
> server on the destination, then the mirroring is done with equivalent of
> the 'virsh blockcopy' command pointing to the NBD server as the
> destination.
>
I will take a look at the code.
>
> >
> > Sorry to take you so much time to answer my questions. I appreciate it
> > deeply. After I solve this issue, I will write a blog and share these
> > knowledge with other people.
>
> The libvirt-users list archives has a lot of questions and posts from
> other users on the same topic; it may also help if you peruse the archives.
>
I tried previously, but I can't find them. I search them directly by
google? Is there anyway to find the related posts in the archive?


Thanks a lot!
Ed


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