How RAID 5 works

 

This topic describes how device parity protection works.

 

Parity set optimization on SCSI adapters

The IOA determines how parity sets are formed. SCSI IOAs allow you to optimize according to availability, capacity, performance, or a balanced version. A parity set optimized for availability offers a greater level of protection, because it allows a parity set to remain functional in the event of a single SCSI bus failure on the IOA. The parity set is formed from at least three disk units of equal capacity each attached to a separate SCSI bus on the IOA. If you optimize by capacity, the IOA tends to create parity sets with a greater number of disk units. You will increase space used for storing user data, but performance may not be as high. If you optimize for performance, the IOA tends to create a parity set with fewer disk units. This should contribute to faster read and write operations, but might also dedicate slightly more disk capacity to storing parity data.

 

Including additional disk units to a parity set

It is possible to include additional disk units of the same capacity in a parity set after device parity protection is initially started. You can include up to two disk units at the same time. However, if three or more disk units are present and eligible for device parity protection, the system requires that you start a new parity set, rather than include them in an existing parity set. In iSeries™ Navigator you can view the properties of each disk unit. If the protection status of a disk unit is unprotected, it is not protected by device parity protection or mirroring and may be eligible to be included in a parity set or to be started in a new parity set. This will also be indicated by the model number which should be 050 (or 060 if it is a compressed disk unit). You can also exclude disks that do not store parity data from a parity set without stopping device parity protection. You can exclude a protected disk unit with a model number, for example 70, 090 or 080 if it is a compressed disk unit, because it is a disk unit that does not store parity data.

When a parity set grows you may want to consider redistributing the parity data. For example you may begin with seven or fewer disk units, but expand to eight or more by including more disk units. When this happens, you can improve the performance on the parity set by stopping parity protection and starting it again. This redistributes the parity data across eight disks rather than four. In general, spreading the parity data across more disk units improves performance.

A write cache is included in the IOA for each parity set to improve performance of interactive write workloads.

If possible, start device parity protection before adding disk units to a disk pool. This significantly reduces the time it takes to start device parity and configure the disk units.

 

Parent topic:

RAID 5 concepts