Cheaper Than Fibre Channel, Faster Than Tape

"SAS and SATA: Cheapo Disk Is Coming!" was the first headline we thought of for this month's Byte and Switch Insider report, which investigates how vendors are integrating Serial-Attached SCSI (SAS) and Serial Advanced Technology Attachment (SATA) drives into their storage arrays.

But that really tells only part of the story. Traditional parallel ATA and SCSI disk are also cheap compared with Fibre Channel, but neither has taken the external storage market by storm.

"SAS and SATA: The Age of Nearline," was our second idea. But again, this is only partially true. SAS will, after all, be as online a medium as parallel SCSI.

How then to characterize the emergence of serial SCSI and ATA interfaces in external storage which, taken together, are rapidly altering the dynamics of the storage networking industry?

Eventually we settled for a tongue twister – "SAS and SATA: Serious About Serial" – and here's why:In a world where Fibre Channel disk is reserved for only the noblest of online, real-time, 24/7 applications, both SATA and SAS have a major role to play in external storage. To put it simply, as users seek value for money, when they can wait a second or two for the data, they will.

Notice we said "a second or two," not "a few minutes," or "a couple of hours." The distinction important because it renders the traditional dichotomy of online disk and offline tape irrelevant. Enter SATA and SAS as a halfway house: cheaper than Fibre Channel, yet faster than tape.

The catch-all term "nearline" is generically used for the class of device emerging in this middle tier. These boxes have been shipping with parallel ATA drives for a couple of years, and they're now moving rapidly to SATA. By 2005, expect to see SAS and SATA in the same box, as their specs deliberately enable both to sit behind the same controller, using the same backplane. At that point, the data you need first, such as index information for a database, will sit on the higher-performing SAS, while the records themselves will reside on SATA.

This tiered approach within the enclosure is a microcosm of the information lifecycle management (ILM) approach to storing data that such heavyweights as EMC Corp. (NYSE: EMC) and Hewlett-Packard Co. (NYSE: HPQ) are evangelizing at the moment. The principle is to store each piece of data according to its value to the enterprise. If it needs to be online and immediately available, give it top-tier storage; if it needs to be kept but rarely seen, put it out to archive; and if it is still accessed occasionally, keep it on nearline.

At the moment, nearline devices tend to be used for jobs such as backup staging – writing initially to disk (in some cases emulating tape), then dribbling the data off to real tape at leisure, without further impact on the corporate LAN.Another common entry point is disaster recovery, where the fact that the disk at the recovery site has a lower performance profile than the one at the production data center is not a problem, as it will only be activated in the event of a disaster.

It was for a disaster recovery application that U.K. engineering firm KBR began using SATA disk. And as solutions architect Nadeem Mir explains in the report, now that it's there and in use, it's hard to resist the temptation to extend its scope to other applications. "As long as you recognise the data-transfer limits on these disks and the subsystem, then everything runs well."

Some of the vendors cited in the report will tell you that SATA will, over time, replace tape entirely – but then predictions of the demise of tape, rather like the rumours of Mark Twain's death, have often been greatly exaggerated. A more likely scenario is that tape will take its place as the medium for so-called deep vaulting, i.e. archives of data that just needs to be there, possibly for legal reasons.

Other vendors will tell you that SATA will move in the other direction, mounting a serious challenge to Fibre Channel disk as it gains more enterprise functions in its second generation. Maybe, but don't hold your breath. What is true, however, is that EMC and Dell Computer Corp. (Nasdaq: DELL) are already offering SATA as primary storage for the SMB space, and that other developments such as SAS, MAID, and FATA (see the report) are adding a granularity to the nearline and online markets.

— Rik Turner, special to Byte and Switch Insider, and Gabriel Brown, Chief Analyst, Byte and Switch Insider0