On Wednesday, December 31, 2014 10:35:04 AM Rich Freeman wrote: > On Wed, Dec 31, 2014 at 10:00 AM, J. Roeleveld <jo...@antarean.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).
Which is what I was afraid of and is what is keeping me from using it. > 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. As a chroot-on-steroids inside a VM, it sounds usable, but not as a replacement for VMs. > 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. Some of the software we deal with can take up to 30 minutes to fully shutdown and re-initialize. (Gotta love those huge enterprise-level BI applications) > 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. Most of the snapshots I take are during the installation and configuration steps. Not many external connections exist during those stages. And the few that do exist generally re-establish themselves when the snapshot is restored. (All nodes of a single instance will be snapshotted near- simultaneously.) -- Joost
