Re: [ovirt-devel] [VDSM] Live snapshot with ceph disks
----- Original Message -----
From: "Michal Skrivanek"
<michal.skrivanek(a)redhat.com>
To: "Nir Soffer" <nsoffer(a)redhat.com>
Cc: "devel" <devel(a)ovirt.org>, "Eric Blake"
<eblake(a)redhat.com>
Sent: Thursday, June 25, 2015 3:44:59 PM
Subject: Re: [ovirt-devel] [VDSM] Live snapshot with ceph disks
On 19 Jun 2015, at 22:40, Nir Soffer wrote:
> Hi all,
>
> For 3.6, we will not support live vm snapshot, but this is a must for the
> next
> release.
>
> It is trivial to create a disk snapshot in ceph (using cinder apis). The
> snapshot
> is transparent to libvirt, qmeu and the guest os.
>
> However, we want to create a consistent snapshot, so you can revert to the
> disk
> snapshot and get a consistent file system state.
>
> We also want to create a complete vm snapshot, including all disks and vm
> memory.
> Libvirt and qemu provides that when given a new disk for the active layer,
> but
> when using ceph disk, we don't change the active layer - we continue to use
> the
> same disk.
>
> Since 1.2.5, libvirt provides virDomainFSFreeze and virDomainFSThaw:
> https://libvirt.org/hvsupport.html
>
> So here is possible flows (ignoring engine side stuff like locking vms and
> disks)
>
> Disk snapshot
> -------------
>
> 1. Engine invoke VM.freezeFileSystems
> 2. Vdsm invokes libvirt.virDomainFSFreeze
> 3. Engine creates snapshot via cinder
> 4. Engine invokes VM.thawFileSystems
> 5. Vdsm invokes livbirt.virDomainFSThaw
>
> Vm snapshot
> -----------
>
> 1. Engine invoke VM.freezeFileSystems
> 2. Vdsm invokes libvirt.virDomainFSFreeze
> 3. Engine creates snapshot via cinder
> 4. Engine invokes VM.snapshot
> 5. Vdsm creates snapshot, skipping ceph disks
> 6. Engine invokes VM.thawFileSystems
> 7. Vdsm invokes livbirt.virDomainFSThaw
>
> API changes
> -----------
>
> New verbs:
> - VM.freezeFileSystems - basically invokes virDomainFSFreeze
> - VM.thawFileSystems - basically invokes virDomainFSThaw
once we do it explicitly we can drop the flag from libvirt api which does it
"atomically" for us right now
We plan to add a "frozen" key in the snapshot description. If ture, it means
that engine freezed the file systems on the guest, and vdsm will skip the
VIR_DOMAIN_SNAPSHOT_CREATE_QUIESCE flag.
also note the dependency on functional qemu-ga (that's no
different from
today, but the current behavior is that when qemu-ga is not running we are
quietly doing an unsafe snapshot)
We plan to keep this behavior, hopefully make it better, depending on
how libvirt communicate this condition.
Current code is assuming that *any* exception calling snapshotCreateXML()
means the guest is not running :-)
Nir