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Why will you like SAS?

Why will you like SAS?

In San Jose last week, Bell Micro, Fujitsu, LSI Logic and Supermicro got together to hold a coming out party for Serial Attach SCSI, or SAS, the newest incarnation of the SCSI device interface.

The offering comes from Bell Micro, which is bundling Fujitsu's 2.5-inch SAS disk drives and LSI's SAS host bus adapters inside eight-bay device canisters from Supermicro. Bell expects the package to appeal to the channel, primarily going into direct attach configurations that require access to lots of data along with high-speed I/O.

The SAS interface offers several advantages. Most obvious is the significant improvement over parallel SCSI when it comes to throughput (about four times the capacity). SAS also allows many more drives to be connected to the SCSI bus: parallel SCSI had room for devices at 15 addresses, but even assuming multiple logical units (LUN) at each address, parallel SCSI could never approach the connectivity that SAS allows - up to 128 addresses using an expander.

Of course, neither parallel SCSI nor serial SCSI could ever access data from so many devices at once - five devices operating at the same time is a number likely to saturate the bus. Still, with proper management, an application can be connected to a lot of data using SAS and then streamed in as appropriate.

Vendors like serial technologies because they are substantially easier to design for than parallel connections. Serial technologies require less bulk when it comes to cabling cable (that means lower costs), and less bulk means less-restricted air flow and thus smaller fans (cheaper again). But the main reason vendors like designing for serial is that the relationship of signals and the system clock is again much simpler. For example, parallel SCSI has to coordinate clock ticks with signals along multiple separate parallel wires but with SAS, all signals travel along a single wire. Controlling timing is easier, and device back planes, connectors, and so forth are easier to design. Oh yes, and cheaper to design as well.

But why will you like SAS? Glad you asked.

Engineers probably think you will like it because it maintains (and extends) the same rich command syntax for manipulating data that SCSI has been using for two decades, while at the same time providing for much faster data transfer rates.

I say you'll like SAS because it goes a long way towards putting you back in control of your storage budgets. Here's why.

The SAS backplane offers you several very nice advantages. One you will get a lot of mileage from is the fact that with a SAS backplane you can plug in either SAS or Serial ATA (SATA) devices. SATA devices will be significantly cheaper than SAS, and in many cases will provide perfectly acceptable performance and reliability. For "nearline" operations such as second-tier storage or use in a virtual tape library, they will certainly be just fine. Many smaller shops that are very price-sensitive will go with them for almost all their storage needs.

But it is important to bear in mind that the SAS devices are tested against a much more rigorous standard than are the SATA drives. Mean time between failure (MTBF) and the duty cycle against which SAS devices are tested distance them from the SATA devices: thus, expect them to last longer and to take more of a beating. For many IT managers this will be an important differentiator.

Also, SAS drives come with a second data port to support redundant data path configurations. Because of this, you can assume that not only are the drives themselves more robust, but because they also provide twice as many data pathways, they supply a higher availability system when the data leaves the drive.

The important point to keep in mind is that you can play mix and match within a device - one bay of drives can be SAS, the next can be SATA, and so forth. You can make your decision on populating with SAS or SATA based on need and economics, and not because you are locked-in to a particularly technology.

Looked at individually, SAS and SATA each offer significant benefits. Taken together, they supply an excellent choice for you as you look to your next round of storage buys.


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