BTRFS: Seed devices as data containers

[frank-dspeed]

see: https://btrfs.readthedocs.io/en/latest/Seeding-device.html

Seed-device
Seed-device use example
The original filesystem and devices are included as a readonly starting point to the new FS. All modifications go onto different devices and the COW machinery makes sure the original is unchanged.

# dd if=/dev/zero of=/mnt/gentoo/btrfs-test-1 count=2 bs=1G
# dd if=/dev/zero of=/mnt/gentoo/btrfs-test-2 count=2 bs=1G
# losetup /dev/loop0 /mnt/gentoo/btrfs-test-1
# losetup /dev/loop1 /mnt/gentoo/btrfs-test-2
# mkfs.btrfs /dev/loop0
# mount /dev/loop0 /mnt/test/
# echo a > /mnt/test/a
# echo b > /mnt/test/b
# umount /mnt/test
Now we will use this filesystem as a seed device:

# [btrfstune](https://web.archive.org/web/20211127174855/https://btrfs.wiki.kernel.org/index.php/Btrfstune) -S 1 /dev/loop0
# mount /dev/loop0 /mnt/test
mount: block device /dev/loop0 is write-protected, mounting read-only
# btrfs device add /dev/loop1 /mnt/test
# ls /mnt/test
a  b
# echo c > /mnt/test/c
bash: c: Read-only file system
# mount -o remount,rw /mnt/test
OR
# umount /mnt/test
# mount /dev/loop1 /mnt/test
# echo c > /mnt/test/c
# echo d > /mnt/test/d
# umount /mnt/test
# mount /dev/loop0 /mnt/test
mount: block device /dev/loop0 is write-protected, mounting read-only
# ls /mnt/test
a  b
# umount /mnt/test
# mount /dev/loop1 /mnt/test
# ls /mnt/test
a  b  c  d
# cat /mnt/test/c
c
It's also worth noting that seed devices can be recursive (Tested on 3.3 with the btrfs/for-linus branch that went into 3.4-final merged in):

# dd if=/dev/zero of=seed1.img bs=1 seek=1G count=1
1+0 records in
1+0 records out
1 byte (1 B) copied, 7.4136e-05 s, 13.5 kB/s

# dd if=/dev/zero of=seed2.img bs=1 seek=1G count=1
1+0 records in
1+0 records out
1 byte (1 B) copied, 5.8166e-05 s, 17.2 kB/s

# dd if=/dev/zero of=seed3.img bs=1 seek=1G count=1
1+0 records in
1+0 records out
1 byte (1 B) copied, 8.4253e-05 s, 11.9 kB/s

# sudo losetup /dev/loop0 seed1.img

# sudo losetup /dev/loop1 seed2.img

# sudo losetup /dev/loop2 seed3.img

# sudo mkdir /mnt/btrseed

# sudo mkfs.btrfs -d single -m single /dev/loop0

WARNING! - Btrfs Btrfs v0.19-dirty IS EXPERIMENTAL
WARNING! - see [http://btrfs.wiki.kernel.org](https://web.archive.org/web/20211127174855/http://btrfs.wiki.kernel.org/) before using

fs created label (null) on /dev/loop0
        nodesize 4096 leafsize 4096 sectorsize 4096 size 1.00GB
Btrfs Btrfs v0.19-dirty

# sudo mount /dev/loop0 /mnt/btrseed

# sudo touch /mnt/btrseed/seed1

# sudo umount /mnt/btrseed

# sudo btrfstune -S 1 /dev/loop0

# sudo mount /dev/loop0 /mnt/btrseed
mount: block device /dev/loop0 is write-protected, mounting read-only

# sudo btrfs dev add /dev/loop1 /mnt/btrseed

# sudo mount -o remount,rw /mnt/btrseed

# sudo touch /mnt/btrseed/seed2

# ls /mnt/btrseed
seed1  seed2

# sudo umount /mnt/btrseed

# sudo btrfstune -S 1 /dev/loop1

# sudo mount /dev/loop1 /mnt/btrseed
mount: block device /dev/loop1 is write-protected, mounting read-only

# sudo btrfs dev add /dev/loop2 /mnt/btrseed

# sudo mount -o remount,rw /mnt/btrseed

# sudo touch /mnt/btrseed/seed3

# ls /mnt/btrseed
seed1  seed2  seed3

# sudo umount /mnt/btrseed

# sudo mount /dev/loop1 /mnt/btrseed
mount: block device /dev/loop1 is write-protected, mounting read-only

# ls /mnt/btrseed
seed1  seed2

# sudo mount /dev/loop0 /mnt/btrseed
mount: block device /dev/loop0 is write-protected, mounting read-only

# ls /mnt/btrseed
seed1