Oracle Solaris ZFS Administration Guide

Replication Features of a ZFS Storage Pool

ZFS provides data redundancy, as well as self-healing properties, in mirrored and RAID-Z configurations.

Mirrored Storage Pool Configuration

A mirrored storage pool configuration requires at least two disks, preferably on separate controllers. Many disks can be used in a mirrored configuration. In addition, you can create more than one mirror in each pool. Conceptually, a basic mirrored configuration would look similar to the following:

mirror c1t0d0 c2t0d0

Conceptually, a more complex mirrored configuration would look similar to the following:

mirror c1t0d0 c2t0d0 c3t0d0 mirror c4t0d0 c5t0d0 c6t0d0

For information about creating a mirrored storage pool, see Creating a Mirrored Storage Pool.

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:

All traditional RAID-5-like algorithms (RAID-4, RAID-6, RDP, and EVEN-ODD, for example) might experience 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 been written to disk, the parity will remain unsynchronized with the data, and therefore forever useless, (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 disk 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

Conceptually, a more complex 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, consider splitting the disks into multiple groupings. For example, a RAID-Z configuration with 14 disks is better split into 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 a RAID-Z Storage Pool.

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

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

ZFS Hybrid Storage Pool

The ZFS hybrid storage pool, available in Oracle's Sun Storage 7000 product series, is a special storage pool that combines DRAM, SSDs, and HDDs, to improve performance and increase capacity, while reducing power consumption. With this product's management interface, you can select the ZFS redundancy configuration of the storage pool and easily manage other configuration options.

For more information about this product, see the Sun Storage Unified Storage System Administration Guide.

Self-Healing Data in a Redundant Configuration

ZFS provides self-healing data in a mirrored or RAID-Z configuration.

When a bad data block is detected, not only does ZFS fetch the correct data from another redundant copy, but it also repairs the bad data by replacing it with the good copy.

Dynamic Striping in a Storage Pool

ZFS dynamically stripes data across all top-level virtual devices. The decision about where to place data is done at write time, so no fixed-width stripes are created at allocation time.

When new virtual devices are added to a pool, ZFS gradually allocates data to the new device in order to maintain performance and disk space allocation policies. Each virtual device can also be a mirror or a RAID-Z device that contains other disk devices or files. This configuration gives you flexibility in controlling the fault characteristics of your pool. For example, you could create the following configurations out of four disks:

Although ZFS supports combining different types of virtual devices within the same pool, avoid this practice. For example, you can create a pool with a two-way mirror and a three-way RAID-Z configuration. However, your fault tolerance is as good as your worst virtual device, RAID-Z in this case. A best practice is to use top-level virtual devices of the same type with the same redundancy level in each device.