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Mix SAS/SATA drives for speed or capacity

Ezine

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Disk-drive differences

The number of disk drives, and the different combinations of how to configure them, is growing steadily. What follows are some key differences among Fibre Channel (FC), SAS and SATA disk drives, and why you may want to choose one over the other.

Controllers. Faster disk drives are only part of the equation for faster performance. You should ensure that the storage array controllers are sized appropriately and configured to adequately deliver the performance the drives are designed to deliver.

Construction. Under the covers, SAS and FC disk drives are constructed the same way with the same capacity, reliability and performance characteristics. Only the disk-drive interface differs between the two. SATA disk drives have higher capacity and lower performance characteristics than enterprise-class drives with comparable reliability ratings.

Firmware. The firmware on SAS and FC disk drives is better able to respond to the differing SCSI commands issued by storage array controllers, thereby providing better responsiveness to commands and more timely error detection and reporting. Conversely, the firmware on SATA disk drives is still relatively immature and may result in a controller incorrectly reporting a disk drive time-out or marking a disk drive as failed.

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Interface. SAS and SATA disk drives each support the 3Gb/sec SAS interface speeds, while FC drives currently (or will shortly) support the 4Gb/sec FC interface speed. A key design concern is that disk drives with SAS or SATA interfaces won't plug into FC sockets or vice versa. In addition, SATA disk drives may deliver 1.5Gb/sec or 3Gb/sec speeds, but they're compatible with 3Gb/sec SAS sockets.

RAID groups. Controllers don't permit the intermixing of different types of disk drives in the same RAID group. You need to ensure that there are sufficient sockets and the right types of disk drives to create the type of RAID group you want.

Early struggles and workarounds
Most vendors are candid about their early struggles and equally forthright about what users need to do to correct the situation. Winchester Systems found that within its first generation of products there were some basic issues that caused disk drives to logically drop out of an array group. In an attempt to fix this serious issue, Winchester Systems tried disk drives from different vendors but experienced the same problems. As a result, for some of its customers who had unresolved issues, Winchester Systems replaced their SAS and SATA storage arrays with FC systems.

This was first published in July 2007

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