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FC and SCSI drive resiliency

In "SAS disk drive advantage?", Greg Schulz mentioned that "SATA disk drives have the characteristic of lower performance, higher capacity and lower resiliency compared to traditional enterprise class Fibre Channel (FC) and SCSI disk drives." What is done at the drive level that gives FC and SCSI drives higher resiliency?

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The simple answer to what enables FC and SCSI disk drives to have higher resiliency compared to high capacity ATA and SATA disks is the way that the disks are designed and built. Enterprise class disk drives that include FC, Parallel SCSI and the emerging SAS disk drives are designed to a higher standard that results in a higher cost point per disk drive. The higher costs are the results of extra technology for performance (e.g. drive based processors and cache), longer duty cycle, sustained multistream I/O, and built-in multipathing.

One of the things that impact the reliability of desktop and high capacity disk drives is their design duty cycle and usage patterns. For example, a typical enterprise class disk drive may be constantly spinning and subjected to sustained I/Os from multiple streams (applications and hosts). These types of disks are designed for these applications to support the rigors of that type of operation. On the other hand, desktop type disk drives are designed for a shorter duty cycle along with being spun up and spun down for power management.

Measuring the MBTF of a disk drive may not be a straight forward comparison. For example, an enterprise class disk may have an MBTF of over a million hours or about double that of desktop class disk drive. Another measure is to look at the number of hours the drives are actually in use compared with the time the disk drives are idle or spun down. In other words, look at the duty cycle time vs. the total elapsed time.

You can learn more about different disk drives and their interfaces from the SCSI Trade Association, the SATA industry trade association, and from disk drive manufacturers (Fujitsu, HGST, Maxtor, Seagate, Western Digital) Web sites.

This was first published in July 2005

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