Recently I was asked to outline the difference between SAS and SATA disks and any finger in the air type points around performance. Ive come to notice that this is rather a common question. It’s well understood that Fibre disks are high performing (perhaps its the inclusion of the name fibre) but there is a general sense of perplexity when it comes to SAS and SATA disks.
The first point to note is that both SAS and SATA are protocol standards maintained by an independent committee – International Committee for Information Technology Standards.
Within this organisation the T10 committee is responsible for the SCSI (and hence SAS [pdf]) and the T13 committee responsible for ATA (and hence SATA [pdf]). Details of the both these protocols and publications can be found on the websites:
So with that …
The speeds at which SATA and SAS operate at are:
SATA (SATA-150) 1500 Mbit/s (187.5 MB/s)
SATA (SATA-300) 3000 Mbit/s (375 MB/s) approx 3Gbps
SAS 3000 Mbit/s (375 MB/s) approx 3Gbps
Note however that by far the most important parameter in assessing overall performance of the hard disk itself is the drive’s spindle speed. This is usually measured in revolutions per minute (RPM).
SATA disks spin at 7,200 RPM
SAS disks spin at either 10,000 RPM or 15,000 RPM
Here is where the performance difference is most noticeable.
To elaborate on this, to achieve a high read/write rate, more data needs to pass over the heads of the disk and this is achieved by a high platter velocity ie the spindle speed (RPM). Access times are also improved with high spindle speeds by reducing the latency of waiting for the sectors to align in the track itself (further assisted by the cache).
Also on the subject of performance; the rule of thumb is that a larger number of spindles perform better for a given capacity.
This is based on the concept of parallelism ie: the ability to access multiple disks simultaneously which allows for data to be written to or read from a RAID array faster than would be possible with a single drive. Different levels of RAID of course lend to a varying performance.
I hope that the above primer will be a decent introduction to SAS and SATA technology. Of course when it comes to performance consideration of an entire subsystem, there are several more metrics that need to be looked at.
