FreeBSD UFS/ZFS Snapshot

http://people.freebsd.org/~rse/snapshot/

Additionally, with the help of md(4) and mdconfig(8) you can mount UFS snapshots as read-only filesystems, too. For illustration purposes, look at this sample session where a snapshot of /var is created on UFS:

# echo "before" >/var/tmp/foo.txt
# ls -l /var/tmp/foo.txt
-rw-r--r-- 1 root wheel 7 Sep 3 16:19 /var/tmp/foo.txt

# test -d /var/.snap mkdir /var/.snap
# mount -u -o snapshot /var/.snap/test /var
# ls -l /var/.snap/test
-r-------- 1 root operator 2147483784 Sep 3 16:19 /var/.snap/test

# echo "after" >/var/tmp/foo.txt
# ls -l /var/tmp/foo.txt
-rw-r--r-- 1 root wheel 6 Sep 3 16:20 /var/tmp/foo.txt

# mdconfig -a -t vnode -f /var/.snap/test -u 1
# mount -o ro /dev/md1 /mnt
# ls -l /mnt/tmp/foo.txt /var/tmp/foo.txt
-rw-r--r-- 1 root wheel 7 Sep 3 16:19 /mnt/tmp/foo.txt
-rw-r--r-- 1 root wheel 6 Sep 3 16:20 /var/tmp/foo.txt
# cat /mnt/tmp/foo.txt /var/tmp/foo.txt
before
after

# umount /mnt
# mdconfig -d -u 1
# rm -f /var/.snap/test


The same session with /var on ZFS could look like this:

# echo "before" >/var/tmp/foo.txt
# ls -l /var/tmp/foo.txt
-rw-r--r-- 1 root wheel 7 Sep 3 16:19 /var/tmp/foo.txt

# zfs snapshot `zfs list -H -o name /var`@test
# ls -ld /var/.zfs/snapshot/test
drwxr-xr-x 23 root operator 23 Sep 3 16:19 /var/.zfs/snapshot/test

# echo "after" >/var/tmp/foo.txt
# ls -l /var/tmp/foo.txt
-rw-r--r-- 1 root wheel 6 Sep 3 16:20 /var/tmp/foo.txt

# ls -l /var/.zfs/snapshot/test/tmp/foo.txt /var/tmp/foo.txt
-rw-r--r-- 1 root wheel 7 Sep 3 16:19 /mnt/tmp/foo.txt
-rw-r--r-- 1 root wheel 6 Sep 3 16:20 /var/tmp/foo.txt
# cat /var/.zfs/snapshot/test/tmp/foo.txt /var/tmp/foo.txt
before
after

# zfs destroy `zfs list -H -o name /var`@test