SCSI and the evolution of a parallel interface
The earliest SCSI specification appeared in the 1980s as a hard disk interface. There have been numerous revisions of the standard that have increased the number of possible connections, and boosted cable length and data transfer rates.
SCSI-2 made some big steps toward standardisation, improved the data transfer rate and supported new types of devices. With the introduction of SCSI-2 came Wide SCSI, which supported up to 15 devices on a single SCSI cable.
SCSI-3, or Ultra SCSI, increased the data transfer rate and brought longer cable lengths and simpler cable schemes. SCSI-3 included various SCSI-3 subtypes such as Fast-20, Fast-40 and so on, which refer to the clock speed (in MHz) of the bus.
A key attribute of SCSI for most of its history is that it has been a parallel (eight bits at a time in a ribbon cable) rather than serial (one bit at a time) interconnect. Parallel cables suffer from 'crosstalk,' which limits cable length and data transfer speed. These limitations have been overcome by the development of SAS.
ATA to SATA
Advanced Technology Attachment (ATA) has been the most popular interface for hard disk drives and CD drives in mainstream systems for many years. This means there are lots of them around and their specifications vary. But you're probably going to need to install one at some point, so it's useful to know what differentiates one ATA drive from another.
The parallel version of ATA has reached its practical limit, but the demand for greater drive speeds won't diminish any time soon. That's the driving force behind the introduction and quick growth in popularity of Serial ATA (SATA).
SATA is a new method of transferring data between the drive and the motherboard (or interface board) that uses a high-speed serial bus instead of a lower-speed parallel one. You would think that dividing the bus width by eight would make it slower, but the speed of the bus is increased by more than eight times, so the net result is a faster interface. Future versions of SATA under development will be even faster.
The inclusion of ATA in the name of this new technology is significant. Inside the drives themselves, things haven't changed much. The original ATA standards at work inside the drive are still intact. The only thing that has changed is the I/O method. This is important because it means the operating systems, BIOS, and disk utilities that you know and love will work just fine with either parallel or serial ATA drives with no special modification.
In addition to increased speed (150 MBps) for the first generation (and hopes of 600 MBps or more in the future with Serial ATA II and Serial ATA III) and the lack of the crosstalk issues inherent in a parallel interface, Serial ATA is self-configuring and fully plug-and-play compatible. That means it will be nearly as easy to configure a new hard disk as it is to plug in a USB device.
And because serial cables aren't subject to the harsh length limits of parallel ribbon cables -- which top out at a length of 40cm -- they can be thinner and up to one metre longer. Therefore, you won't have to worry about rearranging the drives in your system to make a cable reach. Just think: no configuration problems with SCSI settings or master/slave issues, fully hot-pluggable and instantly plug and play.
SAS makes SCSI a serial technology
Serial Attached SCSI (SAS) starts with the widely accepted SCSI interface protocol, but applies SATA technology to produce a new standard that can support up to 300 MBps data transfer with 1,200 MBps promised in future versions.
Further, SAS allows each device to utilise the full bandwidth because there's no splitting of bandwidth for multiple devices on a chain. It allows for special wide-port devices to further increase their bandwidth by using multiple 300 MBps channels simultaneously.
So what's the maximum number of devices per chain? Goodbye to the old 15-device limitation; SAS supports up to 128 direct, point-to-point connections. Further, each of those can employ an expander that will allow it to support multiple devices, for a grand total of up to 16,384 per SCSI interface.
SAS uses the same type of cable as SATA and a very similar connector. The SAS interface can also support SATA devices at 150 MBps. However, SATA interfaces can't support SAS devices. The SAS plug has a key that won't fit a SATA socket.
The significance of including SATA compatibility in SAS is huge. SCSI's primary drawback in the past was that the average motherboard didn't support it because its interface was fairly costly to implement. But since SAS and SATA are so similar, it costs manufacturers very little to support both on the same motherboard, and we should soon see more mainstream systems that offer dual support of SAS and SATA.
SATA, SAS or a bit of both?
SAS disk drives deliver high performance vs. SATA disk drives. But there's a new development that combines both the technologies for a higher value proposition. Compatibility with SATA has become a central component of the SAS feature suite and this unified interface enables IT professionals to achieve performance and capacity objectives with a single SAS infrastructure.
Businesses can save money by deploying a SAS infrastructure -- host bus adapters (HBAs), backplanes, cabling and so forth -- that's initially populated with low-cost, high-capacity SATA drives. As the business expands and greater capacity is needed, more SATA drives can be added. With far greater scalability than a SATA infrastructure, a SAS infrastructure can connect more than 16,000 SAS and/or SATA drives in one domain.
A SAS infrastructure can easily support a multitude of SATA drives with a single SAS HBA port. As more SATA drives and SAS RAID backplanes are added to a storage enclosure, SAS expanders on those backplanes can be cascaded together.
With only a minimal number of ports on the HBA required, and with more ports available on the expanders, a SAS infrastructure becomes increasingly cost effective as the number of drives rises. When a growing organisation moves into online/transactional applications that involve critical data needing high availability to multiple, concurrent users, adding high-performance SAS drives requires no modification to the existing infrastructure.
Not only does a SAS infrastructure (via its compatibility with SATA hard drives) enable IT managers to select the most cost-effective storage solution for any task, it vastly improves SATA's scalability. Simply put, SAS compatibility with SATA establishes a lofty new standard by eliminating the redundancies and inefficiencies of purchasing and maintaining separate infrastructures for high-performance (SAS) and high-capacity (SATA) storage applications.
In addition, the availability of small form factor (SFF) 15K rpm SAS drives and higher-capacity, lower-power 3.5-inch 10K rpm SAS hard drives boosts the SAS/SATA value proposition even more by leading the migration to more space- and energy-efficient storage solutions.