Background Information for Creating RAID 5 Volumes

When you work with RAID 5 volumes, consider the Requirements for RAID 5 Volumes and Guidelines for RAID 5 Volumes. Many striping guidelines also apply to RAID 5 volume configurations. See RAID 0 Volume Requirements.

Requirements for RAID 5 Volumes

• A RAID 5 volume must consist of at least three components. The more components a RAID 5 volume contains, however, the longer read and write operations take when a component fails.

• RAID 5 volumes cannot be striped, concatenated, or mirrored.

• Do not create a RAID 5 volume from a component that contains an existing file system. Doing so will erase the data during the RAID 5 initialization process.

• When you create a RAID 5 volume, you can define the interlace value. If not specified, the interlace value is 16 Kbytes. This value is reasonable for most applications.

• A RAID 5 volume (with no hot spares) can only handle a single component failure.

• When you create RAID 5 volumes, use components across separate controllers, because controllers and associated cables tend to fail more often than disks.

• Use components of the same size. Creating a RAID 5 volume with components of different sizes results in unused disk space.

Guidelines for RAID 5 Volumes

• Because of the complexity of parity calculations, volumes with greater than about 20 percent writes should probably not be RAID 5 volumes. If data redundancy on a write-heavy volume is needed, consider mirroring.

• If the different components in the RAID 5 volume reside on different controllers and the accesses to the volume are primarily large sequential accesses, then setting the interlace value to 32 Kbytes might improve performance.

• You can expand a RAID 5 volume by concatenating additional components to the volume. Concatenating a new component to an existing RAID 5 decreases the overall performance of the volume because the data on concatenations is sequential. Data is not striped across all components. The original components of the volume have data and parity striped across all components. This striping is lost for the concatenated component, although the data is still recoverable from errors because the parity is used during the component I/O. The resulting RAID 5 volume continues to handle a single component failure.

  Concatenated components also differ in the sense that they do not have parity striped on any of the regions. Thus, the entire contents of the component are available for data.

  Any performance enhancements for large or sequential writes are lost when components are concatenated.

• You can create a RAID 5 volume without having to “zero out” the data blocks. To do so, do one of the following:

  • Use the metainit command with the -k option. The -k option recreates the RAID 5 volume without initializing it, and sets the disk blocks to the OK state. This option is potentially dangerous, as any errors that exist on disk blocks within the volume will cause unpredictable behavior from Solaris Volume Manager, including the possibility of fabricated data.

  • Initialize the device and restore data from tape. See the metainit(1M) man page for more information.