Gentoo Archives: gentoo-user

From: "J. Roeleveld" <joost@××××××××.org>
To: gentoo-user@l.g.o
Subject: Re: [gentoo-user] VMs - what technology would you advise?
Date: Wed, 31 Dec 2014 16:18:16
Message-Id: 2463395.EZXZmQMniY@andromeda
In Reply to: Re: [gentoo-user] VMs - what technology would you advise? by Rich Freeman
1 On Wednesday, December 31, 2014 10:35:04 AM Rich Freeman wrote:
2 > On Wed, Dec 31, 2014 at 10:00 AM, J. Roeleveld <joost@××××××××.org> wrote:
3 > > The thing lacking from KVM (and I believe also Containers) is that the
4 > > memory contents are not included in snapshots. Making the snapshots
5 > > basically result in an unclean-shutdown scenario.
6 > > Which is ok-ish as a backup, but not when testing different steps where a
7 > > quick and easy roll-back is often required.
8 >
9 > That is a very good point, and as far as I'm aware container memory
10 > can't be snapshotted (unless you count suspend-to-disk of the entire
11 > host).
12
13 Which is what I was afraid of and is what is keeping me from using it.
14
15 > Processes in containers are really just processes on the host,
16 > and I don't think there is much support in linux for snapshotting a
17 > process. The best I could find was BLCR, but that didn't really seem
18 > too mainstream (maybe it is). Snapshotting of the disk is whatever
19 > you can do at the filesystem level - a container typically just looks
20 > like a chroot as far as the host is concerned - typically you stick it
21 > on lvm or btrfs for snapshotting.
22
23 As a chroot-on-steroids inside a VM, it sounds usable, but not as a
24 replacement for VMs.
25
26 > Now, a big advantage of containers is that startup/shutdown is REALLY
27 > fast. It isn't uncommon for me to run something like "systemctl stop
28 > container ; btrfs su snap container container-back ; systemctl start
29 > container" or something to that effect - often it takes less than a
30 > second to run. Containers are just processes in a separate namespace,
31 > so starting/stopping them is as fast as starting/stopping a service
32 > for the most part. Obviously if your process takes a while to
33 > shutdown and you stop it in a graceful manner then you'll be waiting -
34 > if your process takes a very long time to shutdown/startup then maybe
35 > VM-level snapshotting makes more sense.
36
37 Some of the software we deal with can take up to 30 minutes to fully shutdown
38 and re-initialize. (Gotta love those huge enterprise-level BI applications)
39
40 > Depending on what your VM is doing snapshotting and restoring at the
41 > memory level may not be entirely graceful either - obviously any
42 > external connections are not going to be in the same state when it
43 > resumes.
44
45 Most of the snapshots I take are during the installation and configuration
46 steps. Not many external connections exist during those stages.
47 And the few that do exist generally re-establish themselves when the snapshot
48 is restored. (All nodes of a single instance will be snapshotted near-
49 simultaneously.)
50
51 --
52 Joost

Replies

Subject Author
Re: [gentoo-user] VMs - what technology would you advise? Alan McKinnon <alan.mckinnon@×××××.com>