ZFS

This will be a quick overview on how to manage ZFS snapshots and volumes on a FreeBSD 9.0 system, but it should apply to Solaris as well.

A Few Assumptions

Suppose that my system has the following drives.

1. ada0 WDC WD20EARS-00J99B0
   • 3907029168 512 byte sectors
2. ada2 WDC WD20EARS-00J99B0
   • 3907029168 512 byte sectors
3. ada3 ST2000DM001-9YN164
   • 3907029168 512 byte sectors
4. ada4 ST2000DL003-9VT166
   • 3907029168 512 byte sectors
5. ada5 ST2000DM001-9YN164
   • 3907029168 512 byte sectors

Note: To find out what your disks are by running camcontrol devlist and diskinfo -v *disk* to determine the sector size / sector count.

 diskinfo -v da4
da4
        512             # sectorsize
        2000398934016   # mediasize in bytes (1.8T)
        3907029168      # mediasize in sectors
        0               # stripesize
        0               # stripeoffset
        243201          # Cylinders according to firmware.
        255             # Heads according to firmware.
        63              # Sectors according to firmware.
                    Z1E37XQR    # Disk ident.

Creating a ZFS Volume

When creating a new volume, we want to have it start after 1MB from the start, and end maybe 200MB from the end. Therefore, the total size of the volume will be the size of the disk less 201MB. (See http://www.freebsddiary.org/zfs-with-gpart.php)

From diskinfo -v, we know that:

* Each disk has 3907029168 sectors
* We want to start 1MB in from the start.
** 1MB = 1024*1024 bytes. 
** 1024*1024 bytes / 512 bytes/sector = 2048 sectors
* We want to have 200MB at the end. ie: in total, 201MB of the disk is not part of the partition
** 201*1024*1024 bytes = 201*1024*1024 bytes / 512 bytes/sector = 411648 sectors
** Therefore, the size of the zfs partition is 3906617520 sectors - 411648 sectors

Since our disks contains 3907029168 sectors, we would need to create a partition on each disk that has 3907029168 - 411648 (that is 3906617520) sectors. Note that if you are using different disks with different sizes that you will need to adjust the calculation ( partition size = total sectors - 411648 ) accordingly.

# gpart add -b 2048 -s 3906617520 -t freebsd-zfs -l disk00 ada0
# gpart add -b 2048 -s 3906617520 -t freebsd-zfs -l disk01 ada2
# gpart add -b 2048 -s 3906617520 -t freebsd-zfs -l disk02 ada3
# gpart add -b 2048 -s 3906617520 -t freebsd-zfs -l disk03 ada4
# gpart add -b 2048 -s 3906617520 -t freebsd-zfs -l disk04 ada5

Once created, you should be able to create your new ZFS volume by running:

# zpool create -f storage raidz1 gpt/disk00 gpt/disk01 gpt/disk02 gpt/disk03 gpt/disk04
# zpool status
  pool: storage
 state: ONLINE
 scan: none requested
config:

        NAME            STATE     READ WRITE CKSUM
        storage         ONLINE       0     0     0
          raidz1-0      ONLINE       0     0     0
            gpt/disk00  ONLINE       0     0     0
            gpt/disk01  ONLINE       0     0     0
            gpt/disk02  ONLINE       0     0     0
            gpt/disk03  ONLINE       0     0     0
            gpt/disk04  ONLINE       0     0     0

errors: No known data errors

ZFS Snapshot

A ZFS snapshot is a read-only copy of the file system. It can be listed using the zfs list -t snapshot command.

# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
storage@20120820  31.4G      -  3.21T  -
storage@20120924   134G      -  4.15T  -
storage@20121028  36.2G      -  4.26T  -
storage@20121201  33.2M      -  4.55T  -

To create a new snapshot, run

zfs snapshot storage@today

Note that this will not consume additional space when created. It will however grow in size when data gets modified.

# zfs list -t snapshot
NAME               USED  AVAIL  REFER  MOUNTPOINT
storage@20120820  31.4G      -  3.21T  -
storage@20120924   134G      -  4.15T  -
storage@20121028  36.2G      -  4.26T  -
storage@20121201  33.2M      -  4.55T  -
storage@today         0      -  4.58T  -

Snapshot contents can be accessed through a special .zfs/snapshot/ directory. Each snapshot will contain a read-only copy of the data that existed when the snapshot was taken.

# ls /storage/.zfs/snapshot/
20120820/ 20120924/ 20121028/ 20121201/ today/

To roll back to a specific snapshot, run zfs rollback storage@yesterday. This will restore your volume to the snapshot state.

# zfs rollback storage@yesterday

To delete a specific snapshot, run zfs destroy storage@today. Note that this will not work if other volumes depend on it. eg: If you cloned it as another volume.

# zfs destroy storage@today

Data Integrity

One of the strengths of ZFS its resiliency thanks to its transactional file system. The only way data stored on a ZFS volume to be in an inconsistent state is through hardware failure or some sort of fault with the ZFS implementation. Similar to a fsck on ext file systems, a ZFS scrub provides a way to perform filesystem checking. To initiate a scrub, run:

# zpool scrub storage

Once the scrub process is underway, you can view its status by running:

# zpool status storage
  pool: storage
 state: ONLINE
 scan: scrub in progress since Mon Dec  3 23:54:53 2012
    18.3G scanned out of 6.05T at 211M/s, 8h20m to go
    0 repaired, 0.30% done
config:

        NAME            STATE     READ WRITE CKSUM
        storage         ONLINE       0     0     0
          raidz1-0      ONLINE       0     0     0
            gpt/disk00  ONLINE       0     0     0
            gpt/disk01  ONLINE       0     0     0
            gpt/disk02  ONLINE       0     0     0
            gpt/disk03  ONLINE       0     0     0
            gpt/disk04  ONLINE       0     0     0

errors: No known data errors

To stop a scrub process, run:

# zpool scrub -s storage

Drive Failure

On a drive failure, you will see something similar to:

zpool status
  pool: storage
 state: DEGRADED
status: One or more devices could not be opened.  Sufficient replicas exist for
        the pool to continue functioning in a degraded state.
action: Attach the missing device and online it using 'zpool online'.
   see: http://www.sun.com/msg/ZFS-8000-2Q
 scan: scrub canceled on Tue Jun  4 21:45:26 2013
config:

        NAME                     STATE     READ WRITE CKSUM
        storage                  DEGRADED     0     0     0
          raidz1-0               DEGRADED     0     0     0
            da1p1                ONLINE       0     0     0
            da2p1                ONLINE       0     0     0
            da3p1                ONLINE       0     0     0
            6594991764398825070  UNAVAIL     39   148     0  was /dev/da4p1
            da5p1                ONLINE       0     0     0

errors: No known data errors

The solution to the above is to replace the dead drive (obviously) and re-add and resync the data.

NFS Share

On FreeBSD, to share a ZFS pool over NFS, run the following:

# showmount -e
Exports list on localhost:
# service mountd start
# zfs sharenfs=on storage
# showmount -e
Exports list on localhost:
/storage                           Everyone

The above will create a share /storage that is available to all. If you want to restrict the share to a specific network, you can do something like:

# zfs sharenfs="-network 10.1.1.0/24" storage
# showmount -e
Exports list on localhost:
/storage                           10.1.1.0

By the way, the exports are stored in /etc/zfs/exports and not in the usual /etc/exports. The ZFS and mountd service must be started for it to work. Therefore, you'll also need to append to /etc/rc.conf the following line:

mountd_enable="YES"

Other Notes

For your ZFS pool to be mounted on startup, you will need the zfs service enabled by having the following line in /etc/rc.conf:

 zfs_enable="YES"