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The Truth About Storage Reliability: Page 2 of 2

And get this, the studies found that drive failures in individual systems come in clusters. The CMU group found that drives were significantly more likely to fail within hours when another drive in the same server cluster failed. That's bad news for RAID 5 fans, since drive capacity has grown much faster than data-transfer rates, so even in systems with hot spares, it could be several hours, or longer, before the array is rebuilt. A second drive failure, or unrecoverable read error, during that time would mean lost data.

Although SATA reliability appears to rival that of SCSI and FC, enterprise-class drives--due to their higher rotational speed and faster positioners--provide better performance in random I/O applications. SAS (Serial-Attached SCSI) and FC drives are dual-ported, giving redundant RAID controllers independent paths to the drive so a controller or data-path failure won't result in data loss.

So what can we storage managers take from all this academic research? First, choose your drives based on capacity and performance rather than vendor claims. If your application needs lots of space and will primarily do sequential I/O, go with lower cost SATA drives without feeling guilty about giving up reliability--because you haven't. Second, since the odds of multiple drives failing in an array before the array can be re-built are higher than we imagined, you should investigate RAID-6, data replication or other arrangements that can survive multiple drive failures. Now that vendors from Network Appliance to AMCC are speeding up the parity calculations on their RAID 6 controllers, I'm recommending RAID 6 for all the arrays we build with 500-GB and bigger drives.

Howard Marks is founder and chief scientist at Networks Are Our Lives, a network design and consulting firm in Hoboken, N.J. Write to him at [email protected].