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A seeded image keeps its parent image as a read-only "seed" device that
joins the seeded image in a multi-device btrfs filesystem. That means
that to mount such an image, the parent image must be made available as
a block device (e.g. using losetup, or dd to an existing block device
and run btfrs device scan).
Such support had been added to the Samizdat Makefile, but more
properly belongs here.
This is the sequence of commands used by sami.git's Makefile to
accomplish the effect:
rm -f $@~tmp
touch $@~tmp
fallocate -l $(samizdat_btrfs_patch_size) $@~tmp
test -d $@.mnt || mkdir $@.mnt
! mountpoint -q $@.mnt || umount $@.mnt
mount -o compress,ro -t btrfs $< $@.mnt
a=$(get_loop_dev); [ -z "$$a" ] || losetup -d $$a
losetup -f $@~tmp
btrfs device add $(get_loop_dev) $@.mnt
mount -o compress,rw,remount $@.mnt
The Makefile also defines:
get_loop_dev="$$(sudo losetup -n -O name -j $@~tmp)"
The same basic sequence is carried out here.
The config file format is modified so that to get a seeded image you
specify "seedme: <bytes>" where <bytes> is the size of the seeded image.
This feature is complete enough to create a seeded image (i.e., to
replace the Makefile), but children of a seeded image seem impossible
to work because there is no code yet to run losetup on seed images of
parents. Thus it can be expected that deriving from a parent with a
seed will fail when fsmgr tries to mount the parent.
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