----- Original Message -----
From: "Michal Skrivanek" <michal.skrivanek@redhat.com> To: "Nir Soffer" <nsoffer@redhat.com> Cc: "devel" <devel@ovirt.org>, "Eric Blake" <eblake@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