Managing RAID on Linux

SCSI Versus ATA

Overall, SCSI is a much better choice than ATA, both as a standalone and as part of an array. It allows more devices per channel and provides higher throughput. It also has a much larger command set, compared to ATA, which translates into better performance and increased reliability. The only major drawback of SCSI is price. SCSI drives and controllers are generally more expensive, with SCSI drives typically costing two or three times as much per megabyte as their ATA equivalents. (Although today, some mid-range motherboards are available with built-in SCSI controllers at little extra cost.) If you plan to use external SCSI devices, you will need to spend extra money on cabling and external disk enclosures. On the other hand, ATA does not support external devices at all, so its expandability is limited.

Speed

In the past, SCSI outperformed ATA by leaps and bounds, but ATA has caught up substantially in recent years. Today, ATA disks perform as well as SCSI disks, so speed isn’t as much of a factor as it was just three or four years ago. But, with SCSI, you can populate an I/O channel with enough devices to fully utilize the entire pipe. With ATA, you are really limited to one device per channel if you want decent performance from that device, and that’s not enough to utilize the full pipe when working with the most recent ATA specifications.

High-end SCSI drives have data throughputs of about 40 MB/s. When using Ultra 160 SCSI, you would need three or four drives on ...