Tip

How to get SCSI reliability on a SATA budget

What you will learn from this tip: How to beat the lower reliability ratings of SATA drives, while still saving a few bucks.

Serial ATA (SATA) disk drives combine the low price of IDE drives with longer cabling

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and better adaptability to RAID arrays than their Parallel ATA cousins. As a result, SATA drive arrays are becoming increasingly popular for building inexpensive drive arrays, especially in iSCSI SAN applications.

The big tradeoff is reliability. One of the reasons ATA drives are about a third cheaper than SCSI drives is that they are not built to the standards of SCSI drives. That translates into lower reliability that makes some storage professionals hesitant to use them.

The reliability differences are real. The MTBF for SCSI drives is usually between 1 and 1.5 million hours. For SATA drives, the figure is between 0.6 and 1 million hours. However, the difference tends to be oversold in the RAID/SAN context because the drives are used with redundancy in RAID arrays, i.e. with RAID-5 or RAID-10, and the arrays usually have hot-swappable drives and other high-reliability features. Given installation in a RAID-10 array with hot swappable drives, there is only a small difference between the mean time to data loss between SCSI and SATA drives. According to EquaLogic, a vendor of SATA arrays, a 14-SATA drive RAID-10 array with redundant controllers and cooling, autospares and the ability to hot swap all components will have 0.04 disk failures per TB. A similarly configured array using SCSI drives will have 0.05 failures.

Of course, there's nothing that says that an ATA drive can't be built to SCSI standards. Such a high-quality ATA drive would still be cheaper than an SCSI drive because the interface is inherently cheaper. This has led to a new class of products, high-reliability SATA drives designed for RAID arrays. Some of the first entries in this category are additions to Western Digital's Caviar line with SATA interfaces and a MTBF of 1 million hours. On the street, the drives carry about a 20 percent premium over conventional SATA drives. If you want to deploy a SATA RAID but are still concerned about the reliability of conventional SATA disks, these new disks provide an alternative.

For more information:

Tip: Keep your SANs from crashing

Tip: How to prevent common problems in SATA drives

Tip: How to document a SAN

About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last twenty years he has been a freelance writer specializing in storage and other computer issues.

This was first published in December 2004

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