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SATA in the enterprise

Feb 20, 20035 mins
Data Center

* More about serial ATA

Last week, we initiated our coverage of serial ATA, or SATA. Today we’ll discuss several of the implications for enterprise storage in a question and answer format.

Question:  Does SATA belong in a RAID box?

The answer depends on your expectations for such an array.  Some aspects of SATA drives will not be as manageable as their Fibre Channel or SCSI counterparts. However, if your requirements don’t include a need to twiddle some relatively esoteric dials for optimization and are mostly directed at getting highly reliable storage at a cheaper price, SATA may work just fine.

Organizations that may find such devices to be suitable include companies that can’t afford larger and more sophisticated arrays, remote departments, and enterprises whose data management is sophisticated enough to determine which files belong on high availability machines and which can be offloaded to cheaper devices.

Question: What about the drive’s internal makeup? 

Reliability is a key issue with any storage device.  ATA drives have historically had a lower mean time between failure (MTBF) than SCSI or Fibre Channel drives, but that has nothing to do with the ATA connectivity. Rather, it is a function of the market positioning of the drives.  EIDE is very much a commoditized segment of the storage market.

In order to reach lower price points the drive vendors squeezed as much cost out of their hard disk drives as possible and cheaper parts typically mean earlier failure.  Thus, two 32G-byte drives from the same manufacturer might have near-identical performance characteristics but the more cheaply built disk behind the EIDE interface would be more likely have a component fail than would the more robust SCSI device.

Question:  Can we expect hot-plugability?

The SATA specification supports hot plugging; so this issue will have more to do with the way the array vendors implement the connection.  There is nothing to prevent the use of “hot spares” or swapping out drives on the fly.  As is the case with hot swapping however, connectivity between drive and array will be through a “mid-plane” rather than through a cable to facilitate non-disruptive removal and insertion of drives.

Question:  What about the MTBF problem?

Fortunately, you have a choice here.  The 10K-byte drives will likely have components, performance and MTBF that is identical to what SCSI offers, but with capacity that is only a half to a quarter that of SCSI and Fibre Channel HDDs (so far, the Western Digital 36G-byte drive is the only 10K rpm drive I know of).

Much larger but slower (7200 rpm) drives with a shorter MTBF will be available from Maxtor.  Both are likely to make good sense in arrays, but you will have to decide if your choice is to be driven exclusively by purchase price or if performance is part of the decision.  In either case, arrays using such devices are likely to be significantly cheaper than either SCSI or Fibre Channel.

Question:  What will happen to the prices?

Expect prices for slower SATA drives to curve downwards as more Intel-based motherboards are built with the SATA chipset.  Intel has embraced the standard and it is likely to start shipping boards with native SATA support this year. So it is only a matter of time that the low-end devices will begin to achieve the benefits of high production levels and higher yields for the chipsets they rely on.  Faster devices will likely become cheaper as well, but at a much slower rate.

Question: If they build them, will you come?

This really asks at what price or price-performance point does a SATA array get interesting? My crystal fishbowl offers the following insight: it seems pretty clear that if the SATA array vendors can come in at a price at least 30% lower per gigabyte than SCSI for the higher performance arrays (the ones with the 10K disks), they have a definite play.  Arrays with slower spin speeds will have to come in even lower, of course.  You will have to do an ROI analysis to understand which, if any, is best for you.

Question:  What about Fibre Channel and SCSI?

Today, Fibre Channel and SCSI own almost all the server disk market, with roughly five times as much SCSI storage sold as Fibre Channel storage.  The SCSI players should be nervous about SATA pushing up into their sweet spot in the market, and they are.  Their response has been to form a collaborative agreement for enabling Serial Attached SCSI system-level compatibility with SATA hard disk drives, although the results of this effort are surely a few years off.

The physical compatibility between Serial Attached SCSI and SATA will have all sorts of implications for IT cost efficiency in the future.  Imagine being able to mix-and-match your disks according to the way your business will be using them.  In the mean time, the response of the SCSI community sure seems like a pretty good validation of what the ATA vendors are doing.