Fully Automated RAID Level Selection Techniques for Disk Arrays Page 4
Automated Selection Of RAID Levels
One approach to automating SAN design relies on input -- a workload description and information about the target disk array types and their configuration choices. The other is output -- a design for a SAN capable of supporting that workload.
Automated RAID Level Selection Approaches
There are two main approaches to automatically selecting a RAID level -- tagging and integrated. Let's briefly discuss tagging approaches first:
Tagging approaches perform a pre-processing step to tag stores with RAID levels. Once tagged with a RAID level, a store cannot change its tag, and it must be assigned to an LU of that type. Tagging decisions consider each store and its streams in isolation. Two types of taggers are considered here: rule-based, which examine the size and type of I/Os, and model-based, which use performance models to make their decisions. The former tend to have many ad hoc parameter settings, while the latter have fewer of these settings but also need performance-related data for a particular disk array type. In some cases, the same performance models can be used.
Integrated approaches omit the tagging step and defer the automated choice of RAID level until data-placement decisions are made. This allows the RAID level decision to take into account interactions with the other stores and streams that have already been assigned.
Finally, two variants of this approach are a partially adaptive one, in which the RAID level of an LU is automatically chosen when the first store is assigned to it and cannot subsequently be changed, and a fully adaptive variant, in which any assignment pass can revisit the RAID level decision for an LU at any time during its best-fit search. In both cases, the reassignment pass can still change the bindings of stores to LUs and can even move a store to an LU of a different RAID level. Neither variant requires any ad hoc constants, and both can automatically and dynamically select RAID levels.