Comparing SAS, SATA and Fibre Channel

Today, we’ll compare the new Serial Attached SCSI disk technology to other disk technologies now available.

For the next few years, Fibre Channel and Serial Attached SCSI (SAS) will continue to be the interfaces of choice on the SAN. Right now, the performance of the two is essentially the same (assuming Fibre Channel moves up to 4G bit/sec soon). Within about 18 months, though, SAS will likely double its speed, whereas the Fibre Channel community is still trying to decide whether the next step for the technology is going to be to 8G bit/sec or 10G bit/sec.

For the present, however, it is pretty much a toss-up as to which is better, Fibre Channel or SAS. Those who have Fibre Channel are likely to continue with it. But as parallel SCSI represents about 80% of the enterprise disk market, there is obviously plenty of opportunity for SAS.

But there is another contender to consider.

While SAS was being developed, vendors of the less expensive ATA drive technology were upgrading their offerings as well. Serial ATA (SATA) shares many characteristics with SAS and provides an inexpensive alternative for situations where investment in expensive drives is not warranted. We are very probably going to see lots of mixed SAS-SATA environments.

Major similarities between SAS and SATA are:

* Both types of drive plug into the SAS backplane.

* The drives are interchangeable within a SAS drive bay module.

* Both are long-proven technologies, with worldwide acceptance.

Major differences between the two technologies are:

* SATA devices will be less expensive.

* SATA devices use the ATA command set.

* SAS drives have dual porting capability, faster spindle speeds and lower latencies.

* While both types of drives plug into the SAS backplane, a SATA backplane cannot accommodate SAS drives.

* SAS drives are tested against much more rigid specifications than are SATA drives, and have a significantly longer mean time between failures and duty cycle.

* SAS drives are faster, and offer several features not available on SATA, including variable sector sizes, LED indicators, dual ports and data integrity.

So what is the best choice for you?

The choice between Fibre Channel and SAS is at present a tough one, as performance is going to be pretty much the same on both platforms for the near future. If you are a happy Fibre Channel user, and don’t mind paying the slight price differential, you are likely to continue using it.

If you are looking for more flexibility however, then the SAS-SATA interchangeability is likely to be more appealing. The appeal of this will increase if the idea of tiered storage has value for you (if you are considering the benefits of information lifecycle management, this should be the case).

As for SAS and SATA, when choosing between them follow this rule:

Choose SATA when cost is the most important issue; choose SAS whenever data availability and performance count. If you buy a SAS box to put them in, you can play mix-and-match with SAS and SATA drives to your heart’s content.

For the long run, as I gaze into my crystal fishbowl, here is what I see:

1. On the desktop, SATA (that was easy).

2. For servers, both direct-attach and inside-the-box drives will be SAS.

3. For near-line storage, “best in class” will lose out to “good enough.” SATA wins.

4. For connectivity to SANs, Fibre Channel will continue to lead, but iSCSI will pick up momentum.

5. For disk drives on the SAN, SAS will likely win, but the SAN connection will continue to be Fibre where it is presently Fibre.

Unless Fibre Channel comes out with a low-cost alternative to its present set of drives, it is likely to lose out as a drive connection in the long run.