simple zfs snapshot demo

create a zfs file system on four disks

root@sol10u8 /# zpool create zzz c2t0d0 c2t1d0 c3t0d0 c3t1d0
root@sol10u8 /# zfs create zzz/data42

populate it with 2MB of data

root@sol10u8 /# dd if=/dev/urandom of=/zzz/data42/file2MB count=2048 bs=1024
2048+0 records in
2048+0 records out

verify the used disk space is 2MB

root@sol10u8 /# zfs list -o space
NAME        AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
zzz         7.81G  2.11M         0     23K              0      2.09M
zzz/data42  7.81G  2.02M         0   2.02M              0          0

add 3MB

root@sol10u8 /# dd if=/dev/urandom of=/zzz/data42/file3MB count=3072 bs=1024
3072+0 records in
3072+0 records out

wait a couple of seconds, then verify usage of diskspace
2MB + 3MB = 5MB

root@sol10u8 /# zfs list -o space
NAME        AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
zzz         7.81G  5.12M         0     23K              0      5.10M
zzz/data42  7.81G  5.03M         0   5.03M              0          0

create a snapshot to preserve this 5MB forever

root@sol10u8 /# zfs snapshot zzz/data42@Wednesday

snapshot = read only copy
snapshot = takes almost zero diskspace when created
snapshot = consumes diskspace as data changes
snapshot = because it keeps the original data

verify diskspace used by snapshot

root@sol10u8 /# zfs list -o space
NAME                  AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
zzz                   7.81G  5.12M         0     23K              0      5.10M
zzz/data42            7.81G  5.03M         0   5.03M              0          0
zzz/data42@Wednesday      -      0         -       -              -          -

force 3MB of data changes on the original file system

root@sol10u8 /# dd if=/dev/urandom of=/zzz/data42/file3MB count=3072 bs=1024
3072+0 records in
3072+0 records out

this new 3MB of data was written over 3MB of existing data

wait a couple of seconds!!
then verify disk usage of file system and snapshot

root@sol10u8 /# zfs list -o space
NAME                  AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
zzz                   7.81G  8.15M         0     23K              0      8.13M
zzz/data42            7.81G  8.05M     3.02M   5.03M              0          0
zzz/data42@Wednesday      -  3.02M         -       -              -          -

notice that the snapshot uses 3MB --> it's keeping the original 3MB forever!
notice that the zpool consumes 8MB : 5MB of current data + 3MB snapshot data

copying the snapshot to another file system would total to 5MB
the same 5MB that existed at the time we took the snapshot
3MB inside the snapshot + 2MB of original data still on zzz/data42

add another 4MB of data to the file system

root@sol10u8 /# dd if=/dev/urandom of=/zzz/data42/file4MB count=4096 bs=1024
4096+0 records in
4096+0 records out

wait a couple of seconds
verify that data42 now consumes 9MB
snapshot stays at 3MB
total for the pool is now 12 MB
12MB = 3MB snapshot + 2MB original data + 3MB new data + 4MB new data

root@sol10u8 /# zfs list -o space
NAME                  AVAIL   USED  USEDSNAP  USEDDS  USEDREFRESERV  USEDCHILD
zzz                   7.80G  12.2M         0     23K              0      12.1M
zzz/data42            7.80G  12.1M     3.02M   9.03M              0          0
zzz/data42@Wednesday      -  3.02M         -       -              -          -

this new 4MB data is in a new file, so it does not influence the snapshot

take a look at the file system

root@sol10u8 /# ls -l /zzz/data42/
total 18457
-rw-r--r--   1 root     root     2097152 Aug 25 21:45 file2MB
-rw-r--r--   1 root     root     3145728 Aug 25 21:49 file3MB
-rw-r--r--   1 root     root     4194304 Aug 25 21:51 file4MB

take a look at the snapshot

root@sol10u8 /# ls -l /zzz/data42/.zfs/snapshot/Wednesday/
total 10256
-rw-r--r--   1 root     root     2097152 Aug 25 21:45 file2MB
-rw-r--r--   1 root     root     3145728 Aug 25 21:46 file3MB

notice the difference in the time stamp on the 3MB file!!

use zfs send/receive to copy the snapshot

zfs send zzz/data42@Wednesday | ssh server "zfs receive yyy/data42"

or use cp, tar, cpio, rsync... to backup a snapshot
www.linux-training.be explains these commands

destroy the snapshot

zfs destroy zzz/data42@Wednesday