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
