RAID5 Metadevice Guidelines

• Follow the 20-percent write rule - Because of the complexity of parity calculations, metadevices with greater than about 20 percent writes should probably not be RAID5 metadevice devices. If data redundancy is needed, consider mirroring.

• Drawbacks to a "slice-heavy" RAID5 metadevice - The more slices a RAID5 metadevice contains, the longer read and write operations will take when a component fails.

• RAID5 metadevices cannot be mirrored.

• RAID5 metadevices and striping guidelines - Striping guidelines also apply to RAID5 metadevice configurations. Refer to "Striping Guidelines".

• Use different controllers - When creating RAID5 devices, use slices across separate controllers, because controllers and associated cables tends to fail more often than disks. This practice also improves mirror performance.

• Use same-size slices - Use the same size disk slices. Creating a RAID5 metadevice of different size slices results in unused disk space.

• Interlace value - It is configurable at the time the metadevice is created; thereafter, the value cannot be modified. The default interlace value is 16 Kbytes. This is reasonable for most applications. If the different slices in the RAID5 metadevice reside on different controllers and the accesses to the metadevice are primarily large sequential accesses, then an interlace value of 32 Kbytes might have better performance.

• Concatenating to a RAID5 metadevice - Concatenating a new slice to an existing RAID5 will have an impact on the overall performance of the metadevice because the data on concatenations is sequential; data is not striped across all components. The original slices of the metadevice have data and parity striped across all slices. This striping is lost for the concatenated slice, although the data is still recoverable from errors because the parity is used during the component I/O.

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

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