Preliminary Information for Creating RAID5 Metadevices

• A RAID5 metadevice uses storage capacity equivalent to one slice in the metadevice to store redundant information about user data stored on the remainder of the RAID5 metadevice's slices. The redundant information is distributed across all slices in the metadevice. Like a mirror, a RAID5 metadevice increases data availability, but with a minimum of cost in terms of hardware.

• The system must contain at least three state database replicas before you can create RAID5 metadevices.

• A RAID5 metadevice can only handle a single slice failure.

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

• There are drawbacks to a slice-heavy RAID5 metadevice: the more slices a RAID5 metadevice contains, the longer read and write operations will take if a slice fails.

• A RAID5 metadevice must consist of at least three slices.

• A RAID5 metadevice can be grown by concatenating additional slices to the metadevice. The new slices do not store parity information, however they are parity protected. The resulting RAID5 metadevice continues to handle a single slice failure.

• The interlace value is key to RAID5 performance. 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.

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

• Do not create a RAID5 metadevice from a slice that contains an existing file system. Doing so will erase the data during the RAID5 initialization process.

• RAID5 metadevices cannot be striped, concatenated, or mirrored.