On 11/02/2015 18:08, Edward Young wrote:
Hi all,
I'm investigating the ways to improve the live migration performance
in libvirt. I have a question about the vm live storage migration. the
platform is libvirt + qemu + kvm
If we want to migrate a running vm with its virtual disk images to
another node.
we can use 'virsh migrate ....' commands.
What if this vm has a number of disk-only external snapshots? In the
current version, how can live migrate this vm?
Is it possible to blockcommit the snapshots to the base image and
later perform the migration, without shutting down the running vm?
Or is it possible to iteratively transfer all the snapshots to the
destination and later live migrate only the latest new data?
Thanks in advance.
Best,
Ed
Just a quick answer, others can answer on clever ways to
transfer disk deltas between systems:
- The usual way is to store both the base images and the
deltas on some kind of shared storage accessible from
both hosts involved in the migration, thus eliminating
the need to transfer any disk contents.
- To the extent that the (not changing at that point in
time) base image and deltas for the non-current
snapshot are stored in separate files, they could
simply be transferred with ordinary tools such as cp, dd
or rsync.
- If you freeze execution of the guest during migration
(not usually a preferred thing when wanting to do live
migration), remaining deltas could be transferred with a
simple rsync.
- If you can find a way to make the snapshot commands
store all further deltas around the time (just
before/after) the migration in a separate delta file on
a shared drive (NFS etc.), you could do that, then rsync
all the (now stable) underlying files, then live migrate
the running VM from using the shared delta and one hosts
underlying files to using the same shared delta with the
other hosts (identical) underlying files, then commit
the snapshot delta from the NFS drive to the new host's
copy of the next lower snapshot or base image.
ASCII art illustration of the final tip above:
+--------+
| VM |
+--------+
/ \
/4Migrate\
+---------+ +---------+
| Host1 | | Host2 |
+---------+ +---------+
| \ / |
| 2Snap \ /5Commit|
| +----------+ |
| | TempSnap | | On NFS etc.
| +----------+ |
| / \ |
| / 3rsync \ |
+---------+ +---------+
|Snapshots| |Snapshots|
+---------+ +---------+
| 1copy |
+---------+ +---------+
| Base | | Base |
+---------+ +---------+
Note that disks are only identical during step 4
Enjoy
Jakob
--
Jakob Bohm, CIO, Partner, WiseMo A/S. http://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark. Direct +45 31 13 16 10
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