Example ZVOL

Currently in the ZFS for Linux port the only interface available from user space is the zvol. The zvol allows you to create a virtual block device dataset in a zfs storage pool. While this may not immediately seem like a big deal it does open up some interesting possibilities. For example, your virtual block device is now backed by whatever level of zfs data replication you like (mirror, raidz, raidz2, etc) all with online scrubbing. Your virtual block device also has fast snapshots due to zfs’s copy on write transaction model. This allows you do some interesting things like format an ext2 file system which uses a zvol based block device and mount it on your system. Here’s an example how:

Create the tank zpool containing a raidz vdev spread over 4 devices.

> zpool create tank raidz /dev/sdb /dev/sdc /dev/sdd /dev/sde
> zpool list
NAME   SIZE   USED  AVAIL    CAP  HEALTH  ALTROOT
> tank  1.81T   132K  1.81T     0%  ONLINE  -

Create a 100G block device named fish in the tank zpool.

> zfs create -V 100G tank/fish
> zfs list
NAME        USED  AVAIL  REFER  MOUNTPOINT
tank        100G  1.24T  26.9K  /tank
tank/fish   100G  1.33T  23.9K  -

Partition tank/fish as if it were a normal block device.

> sfdisk /dev/tank/fish << EOF
0,
EOF
> sfdisk -l /dev/tank/fish

Disk /dev/tank/fish: 208050 cylinders, 16 heads, 63 sectors/track
Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls    #blocks   Id  System
/dev/tank/fish1          0+ 208049  208050- 104857199+  83  Linux
/dev/tank/fish2          0       -       0          0    0  Empty
/dev/tank/fish3          0       -       0          0    0  Empty
/dev/tank/fish4          0       -       0          0    0  Empty

Format the new /dev/tank/fish1 partition with ext2 and mount it.

> mkfs.ext2 -q /dev/tank/fish1
> mkdir -p /mnt/tank/fish1
> mount /dev/tank/fish1 /mnt/tank/fish1
> ls /mnt/tank/fish1
lost+found

Take a snapshot of the pristine ext2 filesystem and mount it read-only.

zfs snapshot tank/fish@pristine1
mkdir /mnt/tank/fish@pristine1
mount /dev/tank/fish@pristine1 /mnt/tank/fish@pristine1
ls /mnt/tank/fish\@pristine1
lost+found

Changes made to tank/fish1 do not appear in tank/fish@pristine1

> touch /mnt/tank/fish1/foo
> ls /mnt/tank/fish1/
foo  lost+found
> ls /mnt/tank/fish\@pristine1
lost+found

Enjoy the possibilities!