Introducing lxc-snap

lxc-snap: lxc container snapshot management tool

BACKGROUND

Lxc supports containers backed by overlayfs snapshots. The way this is
typically done is to create a container backed by a regular directory,
then create a new container which mounts the first container’s rootfs
as a read-only lower mount, with a new private delta directory as
its read-write upper mount. For instance, you could

sudo lxc-create -t ubuntu -n r0 # create a normal directory
sudo lxc-clone -B overlayfs -s r0 o1 # create overlayfs clone

The second container, o1, when started up will mount /var/lib/lxc/o1/delta0
as a writeably overlay on top of /var/lib/lxc/r0/rootfs, and use that as its
root filesystem.

From here you can clone o1 to a new container o2. This simply copies the
the overlayfs delta from o1 to o2, and you is done with

sudo lxc-clone -s o1 o2

LXC-SNAP

One of the obvious use cases of these snapshot clones is to support
incremental development of rootfs images. Make some changes, snapshot,
make some more changes, snapshot, revert…

lxc-snap is a small program using the lxc API to more easily support
this use case. You begin with a overlayfs backed container, make some
changes, snapshot, make some changes, snapshot… This is a simpler
model than manually using clone because you continue developing the same
container, o1, while the snapshots are kept away until you need them.

EXAMPLE

Create your first container

sudo lxc-create -t ubuntu -n base
sudo lxc-clone -s -B overlayfs base mysql

Now make initial customizations, and snapshot:

sudo lxc-snap mysql

This will create a snapshot container /var/lib/lxcsnaps/mysql_0. You can actually
start it up if you like using ‘sudo lxc-start -P /var/lib/lxcsnaps -n mysql_0’.
(However, that is not recommended, as it will cause changes in the rootfs)

Next, make some more changes. Write a comment about the changes you made in this
version,

echo “Initial definition of table doomahicky” > /tmp/comment

sudo lxc-snap -c /tmp/comment mysql

Do this a few times. Now you realize you lost something you needed. You can
see the list of containers which have snapshots using

lxc-snap -l

and the list of versions of container mysql using

lxc-snap -l mysql

Note that it shows you the time when the snapshot was created, and any comments
you logged with the snapshot. You see that what you wanted was version 2, so
recover that snapshot. You can destroy container mysql and restore version 2
to it, or (I would recommend) use a different name to restore the snapshot to.

Use a different name with:

sudo lxc-snap -r mysql_2 mysql_tmp

or destroy mysql and restore the snapshot to it using

sudo lxc-destroy -n mysql
sudo lxc-snap -r mysql_2 mysql

When you’d like to export a container, you can clone it back to a directory
backed container and tar it up:

sudo lxc-clone -B dir mysql mysql_ship
sudo tar zcf /srv/mysql_ship.tar.gz /var/lib/lxc/mysql_ship

BUILD AND INSTALL

To use lxc-snap, you currently need to be using lxc from the ubuntu-lxc
daily ppa. On an ubuntu system (at least 12.04) you can

sudo add-apt-repository ppa:ubuntu-lxc/daily
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get install lxc

lxc-snap will either become a part of the lxc package, or will become a
separate package. Currently it is available at
git://github.com/hallyn/lxc-snap. Fetch it using:

git clone git://github.com/hallyn/lxc-snap

Then build lxc-snap by typing ‘make’.

cd lxc-snap
make

Install into /usr/bin by typing

sudo DESTDIR=/usr make install

or install into /home/$USER/bin by typing

mkdir /home/$USER/bin
DESTDIR=/home/$USER make install

Note that lxc-snap is in very early development. It’s usage may
change over time, and as it currently ships a copy of liblxc .h
files it needs, it may occasionally break and need to be updated
from git and rebuilt. Using a package (as soon as it becomes
available) is recommended.
lxc package, or will become a
separate package. Currently it is available at
git://github.com/hallyn/lxc-snap. Fetch it using: