Note - In addition to these generic storage options, see Hot Spare Pools for more information about using Solaris Volume Manager to support redundant devices.

Performance Issues

General Performance Guidelines

When you design your storage configuration, consider the following performance guidelines:

• Striping generally has the best performance, but striping offers no data redundancy. For write-intensive applications, RAID 1 volumes generally have better performance than RAID 5 volumes.

• RAID 1 and RAID 5 volumes both increase data availability, but both volume types generally have lower performance for write operations. Mirroring does improve random read performance.

• RAID 5 volumes have a lower hardware cost than RAID 1 volumes, while RAID 0 volumes have no additional hardware cost.

• Identify the most frequently accessed data, and increase access bandwidth to that data with mirroring or striping.

• Both stripes and RAID 5 volumes distribute data across multiple disk drives and help balance the I/O load.

• Use available performance monitoring capabilities and generic tools such as the iostat command to identify the most frequently accessed data. Once identified, the access bandwidth to this data can be increased using striping, RAID 1 volumes or RAID 5 volumes.

• The performance of soft partitions can degrade when the soft partition size is changed multiple times.

• RAID 5 volume performance is lower than stripe performance for write operations. This performance penalty results from the multiple I/O operations required to calculate and store the RAID 5 volume parity.

• For raw random I/O reads, the stripe and the RAID 5 volume are comparable. Both the stripe and RAID 5 volumes split the data across multiple disks. RAID 5 volume parity calculations are not a factor in reads except after a slice failure.

• For raw random I/O writes, the stripe is superior to RAID 5 volumes.

Optimizing for Random I/O and Sequential I/O

This section explains Solaris Volume Manager strategies for optimizing your particular configuration.

If you do not know if sequential I/O or random I/O predominates on the Solaris Volume Manager volumes you are creating, do not implement these performance tuning tips. These tips can degrade performance if the tips are improperly implemented.

The following optimization suggestions assume that you are optimizing a RAID 0 volume. In general, you would want to optimize a RAID 0 volume, then mirror that volume to provide both optimal performance and to provide data redundancy.

Random I/O

In a random I/O environment, such as an environment used for databases and general-purpose file servers, all disks should spend equal amounts of time servicing I/O requests.

For example, assume that you have 40 Gbytes of storage for a database application. If you stripe across four 10 Gbyte disk spindles, and if the I/O is random and evenly dispersed across the volume, then each of the disks will be equally busy, which generally improves performance.

The target for maximum random I/O performance on a disk is 35 percent or lower usage, as reported by the iostat command. Disk use in excess of 65 percent on a typical basis is a problem. Disk use in excess of 90 percent is a significant problem. The solution to having disk use values that are too high is to create a new RAID 0 volume with more disks (spindles).

Note - Simply attaching additional disks to an existing volume cannot improve performance. You must create a new volume with the ideal parameters to optimize performance.

The interlace size of the stripe does not matter because you just want to spread the data across all the disks. Any interlace value greater than the typical I/O request will do.

Sequential Access I/O

You can optimize the performance of your configuration to take advantage of a sequential I/O environment, such as DBMS servers that are dominated by full table scans and NFS servers in very data-intensive environments, by setting the interlace value low relative to the size of the typical I/O request.

For example, assume a typical I/O request size of 256 Kbyte and striping across four spindles. A good choice for stripe unit size in this example would be: 256 Kbyte / 4 = 64 Kbyte, or smaller.

This strategy ensures that the typical I/O request is spread across multiple disk spindles, thus increasing the sequential bandwidth.

Note - Seek time and rotation time are practically zero in the sequential case. When you optimize sequential I/O, the internal transfer rate of a disk is most important.

In sequential applications, the typical I/O size is usually large, meaning more than 128 Kbytes or even more than 1 Mbyte. Assume an application with a typical I/O request size of 256 Kbytes and assume striping across 4 disk spindles. 256 Kbytes / 4 = 64 Kbytes. So, a good choice for the interlace size would be 32 to 64 Kbyte.