Configuration Planning Guidelines

This section provides a list of guidelines for working with RAID 0 concatenations, stripes, RAID 1 mirrors, RAID 5 volumes, state database replicas, and file systems constructed on volumes.

Concatenation Guidelines

Disk geometry refers to how sectors and tracks are organized for each cylinder in a disk drive. The UFS organizes itself to use disk geometry efficiently. If slices in a concatenated metadevice have different disk geometries, Enhanced Storage uses the geometry of the first slice. This fact may decrease the UFS file system efficiency.


Note - Disk geometry differences do not matter with disks that use Zone Bit Recording (ZBR) because the amount of data on any given cylinder varies with the distance from the spindle. Most disks now use ZBR.

When constructing a concatenation, distribute slices across different controllers and buses. Cross-controller and cross-bus slice distribution can help balance the overall I/O load.

Striping Guidelines

Set the stripe's interlace value correctly.

The more physical disks in a striped metadevice, the greater the I/O performance. (The MTBF, however, will be reduced, so consider mirroring striped volumes.)

Do not mix differently sized slices in the striped volume. A striped volume's size is limited by its smallest slice.

Avoid using physical disks with different disk geometries.

Distribute the striped volume across different controllers and buses.

Striping cannot be used to encapsulate existing file systems.

Striping performs well for large sequential I/O and for random I/O distributions.

Striping uses more CPU cycles than concatenation. However, it is usually worth it.

Striping does not provide any redundancy of data.

Mirroring Guidelines

RAID 5 Guidelines

RAID 5 volumes can withstand only a single device failure.

A mirrored volume can withstand multiple device failures in some cases (for example, if the multiple failed devices are all on the same submirror). A RAID 5 volume can only withstand a single device failure. Striped and concatenated volumes cannot withstand any device failures.

RAID 5 volumes provide good read performance if no error conditions, and poor read performance under error conditions.

When a device fails in a RAID 5 volume, read performance suffers because multiple I/O operations are required to regenerate the data from the data and parity on the existing drives. Mirrored volume do not suffer the same degradation in performance when a device fails.

RAID 5 volumes can cause poor write performance.

In a RAID 5 volume, parity must be calculated and both data and parity must be stored for each write operation. Because of the multiple I/O operations required to do this, RAID 5 write performance is generally reduced. In mirrored volumes, the data must be written to multiple mirrors, but mirrored performance in write-intensive applications is still much better than in RAID 5 volumes.

RAID 5 volumes involves a lower hardware cost than mirroring.

RAID 5 volumes have a lower hardware cost than mirroring. Mirroring requires twice the disk storage (for a two-way mirror). In a RAID 5 volume, the amount required to store the parity is: 1/#-disks.

RAID 5 volumes can not be used for existing file systems.

You can not encapsulate an existing file system in a RAID 5 volume (you must backup and restore).

State Database Replica Guidelines for Performance