All my data is currently saved in a quite trustable raidz pool, but this is not theft-proof for example. For increased safety, I chose to externalize a backup of the pool
with a used tape drive on another hard drive.
First, connect the backup disk to the computer, and make sure the device descriptor points to the correct drive :
# smartctl -i /dev/ad2 smartctl 5.42 2011-10-20 r3458 [FreeBSD 8.3-RELEASE-p3 amd64] (local build) Copyright (C) 2002-11 by Bruce Allen, http://smartmontools.sourceforge.net === START OF INFORMATION SECTION === Model Family: Seagate Barracuda Green (Adv. Format) Device Model: ST2000DL003-9VT166 Serial Number: xxxxxxx LU WWN Device Id: 5 000c50 044b0747a Firmware Version: CC3C User Capacity: 2,000,398,934,016 bytes [2.00 TB] Sector Sizes: 512 bytes logical, 4096 bytes physical Device is: In smartctl database [for details use: -P show] ATA Version is: 8 ATA Standard is: ATA-8-ACS revision 4 Local Time is: Wed Jun 20 19:04:15 2012 CEST SMART support is: Available - device has SMART capability. SMART support is: Enabled
Then, create a new pool on this drive, with no particular option :
# zpool create backup /dev/ad2 # zpool status backup pool: backup state: ONLINE scan: none requested config: NAME STATE READ WRITE CKSUM backup ONLINE 0 0 0 ad2 ONLINE 0 0 0 errors: No known data errors
This is followed by the datasets creation. The option copies=2  is used for the “important” datasets in order to protect them against an hypothetical bit rot  (but this double the used disk space).
# zfs create backup/fichiers # zfs create -o checksum=sha256 -o copies=2 backup/fichiers/Photos
Less critical datasets can be created in a more usual way :
# zfs create -o checksum=sha256 backup/fichiers/Videos
For my backups, I chose the
send option  included in ZFS. This feature is mainly used for data replication between hosts, but it is not recommended for backups on a external storage support, for a good reason : if one block is corrupted in the flow, the whole backup becomes unusable, which explains the importance of not forgetting the copies=2 option previously mentioned.
These backups are made from existing snapshots and encrypted with openssl this way :
# zfs send data/fichiers/Photos@062012 | openssl enc -aes-256-cbc -salt > /backup/fichiers/Photos/Photos_062012.ssl
And for “compressible” data :
# zfs send data/fichiers/Documents@062012 | compress | openssl enc -aes-256-cbc -salt > /backup/fichiers/Documents/Documents_062012.z.ssl
Once the backup is finished, the pool can be exported :
# zfs export backup
To make sure the backup is usable, the drive must be checked from time to time, for example with the live CD mode integrated into FreeBSD 9.0 (in this mode, the keymap can be changed via the
A global check (
zpool scrub) is possible after a drive import. All available pools can be listed with the
zpool import command, and an alternative mountpoint can be set with the
-R option, since / is in read-only mode when the live CD is used :
# zpool import # mkdir /tmp/zfsroot # zpool import -R /tmp/zfsroot <pool> # zpool scrub <pool>
To check backups integrity at the filesystem level, the pool can be mounted in read-only, and checked with
zstreamdump  :
# zpool import -o readonly=on -R /tmp/zfsroot <pool> # openssl enc -d -aes-256-cbc -in /tmp/zfsroot/path/to/file.ssl | zstreamdump
zstreamdump returns only the checksum, the backup data is valid. Otherwise, the command returns the following error :
Expected checksum differs from checksum in stream.
 : http://docs.oracle.com/cd/E19253-01/820-2315/gevpg/index.html
 : http://www.linux-mag.com/id/8794/
 : http://docs.oracle.com/cd/E19963-01/html/821-1448/gbchx.html
 : http://blog.richardelling.com/2009/10/check-integrity-of-zfs-send-streams.html