RAID-Z Storage Pool Configuration

In addition to a mirrored storage pool configuration, ZFS provides a RAID-Z configuration with either single, double, or triple parity fault tolerance. Single-parity RAID-Z (raidz or raidz1) is similar to RAID-5. Double-parity RAID-Z (raidz2) is similar to RAID-6.

For more information about RAIDZ-3 (raidz3), see the following blog:

http://blogs.sun.com/ahl/entry/triple_parity_raid_z

All traditional RAID-5-like algorithms (RAID-4, RAID-6, RDP, and EVEN-ODD, for example) suffer from a problem known as the “RAID-5 write hole.” If only part of a RAID-5 stripe is written, and power is lost before all blocks have made it to disk, the parity will remain out of sync with the data, and therefore useless, forever (unless a subsequent full-stripe write overwrites it). In RAID-Z, ZFS uses variable-width RAID stripes so that all writes are full-stripe writes. This design is only possible because ZFS integrates file system and device management in such a way that the file system's metadata has enough information about the underlying data redundancy model to handle variable-width RAID stripes. RAID-Z is the world's first software-only solution to the RAID-5 write hole.

A RAID-Z configuration with N disks of size X with P parity disks can hold approximately (N-P)*X bytes and can withstand P device(s) failing before data integrity is compromised. You need at least two disks for a single-parity RAID-Z configuration and at least three disks for a double-parity RAID-Z configuration. For example, if you have three disks in a single-parity RAID-Z configuration, parity data occupies space equal to one of the three disks. Otherwise, no special hardware is required to create a RAID-Z configuration.

Conceptually, a RAID-Z configuration with three disks would look similar to the following:

raidz c1t0d0 c2t0d0 c3t0d0

A more complex conceptual RAID-Z configuration would look similar to the following:

raidz c1t0d0 c2t0d0 c3t0d0 c4t0d0 c5t0d0 c6t0d0 c7t0d0 raidz c8t0d0 c9t0d0 c10t0d0 c11t0d0
c12t0d0 c13t0d0 c14t0d0

If you are creating a RAID-Z configuration with many disks, as in this example, a RAID-Z configuration with 14 disks is better split into a two 7-disk groupings. RAID-Z configurations with single-digit groupings of disks should perform better.

For information about creating a RAID-Z storage pool, see Creating RAID-Z Storage Pools.

For more information about choosing between a mirrored configuration or a RAID-Z configuration based on performance and space considerations, see the following blog:

http://blogs.sun.com/roller/page/roch?entry=when_to_and_not_to

For additional information on RAID-Z storage pool recommendations, see the ZFS best practices site:

http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide