Re: [gentoo-user] VMs - what technology would you advise?

[Rich Freeman]

On Wed, Dec 31, 2014 at 10:00 AM, J. Roeleveld <joost@××××××××.org> wrote:
>
> The thing lacking from KVM (and I believe also Containers) is that the memory
> contents are not included in snapshots. Making the snapshots basically result
> in an unclean-shutdown scenario.
> Which is ok-ish as a backup, but not when testing different steps where a quick
> and easy roll-back is often required.

That is a very good point, and as far as I'm aware container memory
can't be snapshotted (unless you count suspend-to-disk of the entire
host).  Processes in containers are really just processes on the host,
and I don't think there is much support in linux for snapshotting a
process.  The best I could find was BLCR, but that didn't really seem
too mainstream (maybe it is).  Snapshotting of the disk is whatever
you can do at the filesystem level - a container typically just looks
like a chroot as far as the host is concerned - typically you stick it
on lvm or btrfs for snapshotting.

Now, a big advantage of containers is that startup/shutdown is REALLY
fast.  It isn't uncommon for me to run something like "systemctl stop
container ; btrfs su snap container container-back ; systemctl start
container" or something to that effect - often it takes less than a
second to run.  Containers are just processes in a separate namespace,
so starting/stopping them is as fast as starting/stopping a service
for the most part.  Obviously if your process takes a while to
shutdown and you stop it in a graceful manner then you'll be waiting -
if your process takes a very long time to shutdown/startup then maybe
VM-level snapshotting makes more sense.

Depending on what your VM is doing snapshotting and restoring at the
memory level may not be entirely graceful either - obviously any
external connections are not going to be in the same state when it
resumes.

--
Rich