The init layer needs to be topmost to make sure certain files
are always there (for instance, the ubuntu:12.10 image wrongly
has /dev/shm being a symlink to /run/shm, and we need to override
that). However, previously the devmapper code implemented the
init layer by putting it in the base devmapper device, which meant
layers above it could override these files (so that ubuntu:12.10
broke).
So, instead we put the base layer in *each* images devmapper device.
This is "safe" because we still have the pristine layer data
in the layer directory. Also, it means we diff the container
against the image with the init layer applied, so it won't show
up in diffs/commits.
Right now this does nothing but add a new layer, but it means
that all DeviceMounts are paired with DeviceUnmounts so that we
can track (and cleanup) active mounts.
This is a module that uses the device-mapper create CoW snapshots
You instantiate a DeviceSetDM object on a specified root (/var/lib/docker),
and it will create a subdirectory there called "loopback". It will
contain two sparse files which are loopback mounted into
a thin-pool device-mapper device called "docker-pool".
We then create a base snapshot in the pool with an empty filesystem
which can be used as a base for docker snapshots. It also keeps track
of the mapping between docker image ids and the snapshots in the pool.
Typical use of is something like (without error checking):
devices = NewDeviceSetDM("/var/lib/docker")
devices.AddDevice(imageId, "") // "" is the base image id
devices.MountDevice(imageId, "/mnt/image")
... extract base image to /mnt/image
devices.AddDevice(containerId, imageId)
devices.MountDevice(containerId, "/mnt/container")
... start container at /mnt/container
