Using partman-auto-loop
-----------------------

This allows preseeding of a number of filesystems loop-mounted from images
on a pre-existing filesystem. (At present, there is no way to create the
host filesystem in the same installation run; the primary use case is
installing on filesystem images hosted on a Windows filesystem, in order to
avoid repartitioning the disk.)

The preseedable question partman-auto/disk should be set to the device name
for the host disk (the disk on which filesystem images should be created),
partman-auto/method should be set to "loop", and partman-auto-loop/partition
should be set to the number of the host partition. On modern Linux kernels,
the disk will typically be /dev/sda and the partition number will be 1 for
the first partition on the first hard disk; the disk might be /dev/hda
instead if the kernel still uses the old drivers/ide/ tree for your disk, or
/dev/sdb etc. for subsequent disks.

The preseedable question partman-auto-loop/recipe should consist of
space-separated recipes, each of which are of the form:

  <imagepath> <minsize> <priority> <maxsize> <fstype> <specifiers> .

This is deliberately analogous to the regular partman recipe format as
documented in installer/doc/devel/partman-auto-recipe.txt, so a valid recipe
might look like this (without the line breaks, which have been added for
readability):

  /disks/root.disk 500 10000 5000 ext3
    method{ format }
    format{ }
    use_filesystem{ }
    filesystem{ ext3 }
    mountpoint{ / } .

  /disks/swap.disk 96 512 300% linux-swap
    method{ swap }
    format{ } .

Internal specifiers such as $primary{ } and $bootable{ } may be used as in
normal recipes, but are currently ignored.

Image paths may not contain spaces.

Future work
-----------

partman-auto-loop is in many ways a temporary workaround. It would be
possible to integrate much of this into partman proper, and some of the
steps along the way would be valuable elsewhere.

  * Add multi-device support to partman-auto, in order that one can specify
    partitions on more than one disk in a single recipe. (Note that
    loop-mounted filesystem images are considered, more or less, as
    "disks".) Ensure that recipes are processed in order to the extent that
    one recipe line may depend for its underlying device on a previous
    recipe line having been processed. This naturally has some complications
    in terms of things like calculating desired partition sizes up-front.

  * Make it possible for recipes to specify existing partitions to be
    located (by device path or perhaps by other more stable means),
    modified, and mounted. This would make it possible to specify the host
    filesystem in a more stable way.

  * Add recipe syntax to instruct partman to create loop-mounted filesystem
    images on which other filesystems can be created. This should be done
    with consideration for other complex devices such as LVM, RAID, and
    encrypted devices.

  * Potentially add UI for loop-mounting. See also partman-crypto.

 -- Colin Watson <cjwatson@ubuntu.com>, for Canonical Ltd.