Accommodating arrays

Users need to carefully weigh the pros and cons of using RAID 5 with anything other than FC disk drives. While both FC and SATA drives advertise similar mean time between failure (MTBF) rates of around 20,000 hours (just over two years), these aren't apples-to-apples comparisons. The MTBFs for FC drives are based on rigorous 24-hour duty cycles, whereas SATA drive MTBF rates are based on two- to four-hour duty cycles. So in environments where SATA drives experience more extensive use or are in more performance-intensive environments, higher disk failure rates can be expected.

Fortunately, vendors of SATA-based arrays recognize this deficiency and provide RAID configurations that help to overcome SATA limitations. For instance, EqualLogic's PeerStorage 100E doesn't even give users the option to implement a basic RAID 5 configuration, offering only RAID 10 and 50 options. Behind each of its controllers, EqualLogic puts a total of 14 SATA disks, of which only 12 are active. The 12 active disks are split into two separate groups of six. Each group of six is then configured in a 5 + 1 configuration, so that any of the six disks in either group can fail without data loss. In the event of a disk failure, one of the two passive disks becomes a member in that parity group.

The RAID 10 and RAID 50 configurations EqualLogic offers increase the level of data protection. RAID 10 will perform better for read-intensive applications because it mirrors

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the data on the first group of six disks to the second group of six disks, so that one group or the other may be lost without impact. This level of data protection is expensive because only 1.5TB, or 43% of the available 3.5TB, on each controller is usable. A better option for EqualLogic arrays is to choose its RAID 50 option. This configuration stripes data across both groups of six disks, yielding a more reasonable 2.5TB of usable storage, or 71% of the 3.5TB total, and will provide adequate performance in most environments.

While users find the high capacities and low price points of SATA arrays appealing, they need to consider how these disks get cooled, as well as the impact and difficulty of replacing a disk in the RAID configuration when it fails. For instance, many SATA vendors use a vertical midplane architecture in an attempt to pack as many disks into a blade as possible to maximize capacity. But this approach results in active disks running at the high end of their optimal temperature range, and when they inevitably fail, they're difficult to replace. For instance, Storage Technology Corp.'s (StorageTek) BladeStore array requires an entire SATA blade to be taken offline to replace a single disk, requiring either downtime for all applications using data on that blade or, to avoid downtime, moving all of the data on the blade to another blade on the array to replace the faulty disk.

This was first published in November 2004

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