Hello Everyone,
I've been trying to come up with a solution for libvirt snapshots to fix the
issue with snapshotting when a volume is attached:
https://bugs.launchpad.net/nova/+bug/946830
The main issue here is that calling snapshot in libvirt makes an internal
snapshot of the entire vm, which a) doesn't work for attached volumes b) wastes
a bunch of space while snapshotting memory and ephemeral disks which aren't
used.
There are two potential approaches to solving the issue, and I've prototyped
them below. I need feedback on which approach is better.
OPTION A --> snapshot using qemu-img
This method shuts down the vm and uses qemu-img to create the snapshot in the
disk image
Pros:
works with older versions of libvirt
Cons:
shutting off the vm during snapshotting is overkill and annoying
Caveats:
if it is safe to create disk file snapshots while libvirt has a file handle
open, i can use suspend/resume which is better than managedSave.
If it is safe to delete snapshots while the disk is being written to, i can
resume sooner, minimizing pause time
if it is additionally safe to create snapshots while the disk is being written
to, we can avoid pausing the vm altogether! (sounds dangerous though)
https://github.com/vishvananda/nova/blob/fix-libvirt-snapshot-old/nova/virt/libvirt/connection.py#L619
OPTION B --> libvirt 9.5 snapshots
This method uses the newer snapshot xml in libvirt 9.5 to snapshot only the
root disk.
Pros:
plays nicely with libvirt, so the vm is only paused for the minimum amount of
time
Cons:
requires libvirt 9.5, which doesn't exist in oneiric
Caveats:
This code is untested and a couple tests don't pass yet because I haven't made
an oneiric vm. I want to make sure this is the right approach before I go
through the hassle of updating.
https://github.com/vishvananda/nova/blob/fix-libvirt-snapshot/nova/virt/libvirt/connection.py#L619
So I could use some specific feedback from kvm/libvirt folks on the following
questions:
a) is it safe to use qemu-img to create/delete a snapshot in a disk file that
libvirt is writing to.
if not:
b) is it safe to use qemu-img to delete a snapshot in a disk file that libvirt
is writing to but not actively using.
if not:
c) is it safe to use qemu-img to create/delete a snapshot in a disk file that
libvirt has an open file handle to.
And I could use input from the community on which of the approaches above to
use:
Do we standardize on libvirt 9.5+? or do we use the compatible version that
causes a bigger outage during the snapshot?
Ideal for me would be that at least b) above is true and we can get by with the
compatible version.
Vish
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