Dual-port SATA for higher iSCSI reliability

iSCSI SANS are less reliable than FC SANS, but implementing one with an FC-like architecture and levels of redundancy removes most of the reliability concerns about iSCSI.

What you will learn from this article: Using dual-port SATA devices in your iSCSI SANs can significantly increase reliability by providing redundant access to disks and arrays.

Everyone knows that iSCSI SANS are less reliable than Fibre Channel (FC) SANs. This has less to do with the actual technology than with the way it has been implemented. Implementing an iSCSI SAN with a FC-like architecture and levels of redundancy removes most of the reliability concerns about iSCSI.

More information on SATA devices and iSCSI

Preventing common SATA drive problems

The role of SATA drives debated

iSCSI arrays: Advantages and disadvantages

Most of the concerns about iSCSI reliability center on the use of low-cost SATA drives with lower reliability than SCSI drives. (Although SATA is not part of iSCSI, many storage professionals link them together because SATA drives and arrays are often used to hold down the cost of an iSCSI SAN.)

Since iSCSI SANs, like their FC cousins, usually rely on RAID arrays, the reliability of SATA drives in an appropriately designed array with an appropriately chosen RAID level, is usually of little practical significance. This is especially true if the array features hot swappable spares. A number of vendors, such as Nexsan Technologies, are offering such arrays with iSCSI connectivity.

The major remaining issue is redundant data paths. Dual porting offers higher reliability by providing a redundant path for data in case of failure. It is pretty much standard on FC storage devices. Increasingly, dual-port arrays and drives are becoming available for SATA.

Some arrays, such as Promise Technology Inc.'s VTrak 15200, come with a dual-port host interface built in. Some SATA drives, such as IBM's xSeries Storage 250 GB, also come with dual ports. Devices such as Broadcom Corp.'s BCM5771 bridge allows dual-porting SATA drives.

Not all iSCSI SANs need dual porting. In many iSCSI applications, cost is more important than ironclad reliability, and the relatively small risk of a failure at the port can be neglected. However, as more enterprises come to rely on iSCSI SANs as their main storage technology, there is a growing demand for products like dual-port SATA devices.

About the author: Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in storage and other computer issues.

This was first published in January 2005

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