SATAIII AND SAS 6G

SATAI, SATAII and now SATAIII, what’s the deal?

  SATAIII is the latest and greatest in SATA bus specifications allowing performance increases beyond the capabilities of SATAII, primarily benefitting Solid State Devices (SSD’s). SATAIII is the specification by which all SATAIII compliant devices must operate including hard drives, cabling, optical drives, controllers, SSD’s etc. The SATAIII Specification allows growth in performance and capabilities of the SATA bus and its attachable devices while maintaining compatibility across the industry.

  SATAIII is also referred to as 6Gb⁄s, or 600 MB⁄s. The bus is capable of devices up to 600 MB⁄s, in theory. In reality we usually can’t achieve performance quite that fast. For instance the SATAII bus was designed to be capable of 300 MB⁄s. In real world testing the best we could ever accomplish was 230 MB⁄s with some fast drives in a RAID. I expect we’ll see an actual performance threshold well below the 600 MB⁄s, but much higher than SATAII would allow.

There are three SATA bus specifications, only two of which you’re likely to see these days.

SATAI = 150 MB⁄sec = 1.5 Gb⁄sec
SATAII = 300 MB⁄sec = 3 Gb⁄sec
SATAIII = 600 MB⁄sec = 6 Gb⁄sec

  The higher SATA specification does not necessarily mean the specific drive will be faster than an drive with an older specification. Astounding leaps in drive performance have been achieved in recent years due primarily to platter density. There are few factors that greatly effect drive performance capabilities, platter density, rotational speed and the number of platters a drive contains being foremost.

Standard Hard Drives

  Drive performance is directly related to how fast you can get data bits under the read head. The faster you spin the drive platter (think of it as a CD) the more bits of data pass under the read head in a given period of time. Another variable is the number of platters, the more platters being read or written to at the same time increases the drive performance. Pretty simple so far. Increasing platter density, essentially squeezing more bits into the same space allows more data to be transferred per drive revolution. There are other factors involved but the physics of the drive itself play the largest role in drive performance.

  SATA specification based functions also allow for performance increases. The newer SATA devices have features sets that enable a drive that’s attached to an advanced bus to perform better. Most of these drives are backward compatible with the older bus styles but the older bus styles will not be able to operate those features.

Solid State Hard Drives

  Solid State Devices break the rules. They don’t have have any moving parts and their performance limitations have a lot to do with the number of chips that make up the storage media, speed of those chips and how they’re all interconnected. SSD’s are quickly gaining momentum in the mainstream workplace due to their higher performance and instant data accessability. We’re still in the developement phase of SSD implimentation. SSD performance is getting faster and faster while the drives are getting larger and less expensive. The longer you can wait before you adopt SSD’s the faster and larger storage you will get for your dollar. The move to the SATAIII specification is very helpful for SSD’s since they were getting to the point of saturating the SATAII bus capabilities. SATAIII benefits SSD’s most of all.

Port Multiplication

  We have SATAIII and SAS 6G RAID Controller cards available and most phases of high end storage have moved toward the SATAIII specification. One place that we’re still waiting is with the use of Port Multiplication and less expensive add on storage enclosures. The SATAIII Port Multiplier capable chipsets haven’t been released to the market from developement yet. We expect that to occur soon and expect that we’ll see some expanded capabilities at that time.

  Port Multiplication is where we’ll really see the big performance benefits from SATAIII. A RAID on current SATAII Port Multiplier bus is limited to around 230 MB⁄sec per single cable or port on a SATA host card. This limit is the maximum speed we have seen through a single SATAII port under any configuration (not quite the 300 MB⁄sec advertised for SATAII buses). With the introduction of SATAIII we should experience a doubling of that performance (from 230 to nearly 450 MB⁄sec) on a set of drives attached to a single port multiplier.

  Today we need 2 port multiplier boards and 2 connections to achieve throughputs of over 230 MB⁄sec. Tomorrow, with a SATAIII bus and SATAIII port multiplier boards, we will be able to double that to nearer 450 MB⁄sec for a single eSATA cable.

  This will be a significant improvement to storage performance. But it also will require that not only is the host card SATAIII specification with eSATA ports capable of supporting port multiplication at SATAIII speeds - but also one piece of the puzzle not yet even being talked about: SATAIII port multiplier boards. Until they come available this is all just hopeful talk. I have yet to hear of any progress being made towards design and release of a SATAIII port multiplier chipset. We keep asking though.

updated 4-1-11