Under Development

Serial Attached SCSI

As parallel storage technologies approach their limits, the storage industry has begun a transition to next generation serial technologies. Serial Attached SCSI (SAS) is tipped to be the successor of SCSI and will be extensively used in workstations, servers and external storage boxes, says Venkatesh Ganesh

SCSI is an acronym for Small Computer Systems Interconnect. It is indeed ironic that this attribute (small) hasn’t been applied to SCSI in the true sense for more than a decade. In fact, this protocol—originally designed for disks attached to stand-alone workstations—now provides data to the biggest machines in large enterprises.

Serial evolution

Serial Attached SCSI (SAS) benefits from a number of advances over its parallel predecessor. Firstly, it leverages the 20-year evolution of the SCSI protocol, carrying forward features such as reliability and stability. Secondly, Serial Attached SCSI gives customers a choice by offering compatibility with both Serial ATA disk as well as Serial Attached SCSI disk drives.

Another obvious advantage of SAS is the need for smaller cabling. The cable and connector are smaller, which results in significantly smaller form factor disk drives that can be made available. This allows vendors to significantly upgrade the storage density (in terms of number of devices) in each bay of a storage array. These smaller cables are easier to route around the inside of a chassis, and the serial interface allows for much longer runs of cable as compared to the parallel interface.

SAS will also allow connecting of up to 128 devices on a single bus. This is significantly more than parallel, which allows a theoretical maximum of only 15 devices (15 disks and a host adapter). However, in reality parallel never offered the bandwidth to support that many streaming devices on a single bus.

What is particularly interesting about SAS though, is the fact that these devices will be compatible with SATA devices, thereby allowing a combination of SATA disks with SAS devices.

Mission Serial

Recently, two storage groups announced a joint effort that would enable next generation SAS drives to collaborate with Serial ATA (SATA) drives. The SCSI Trade Association, (which promotes SCSI drive technology) and the Serial ATA Working Group are planning to allow SAS interface technology to use SATA drives. IDC estimates that SAS drives will account for about 50 percent of the market by 2005. SAS will be used in servers running mission-critical applications. Also, in some cases, analysts opine that Serial Attached SCSI can act as an alternative to deploying Fibre Channel storage area networks (SAN) in price-sensitive situations.

While SCSI still dominates the mainstream server storage market, Serial Attached SCSI can provide the extra value for enterprise applications where reliability, availability and scalability are key requirements. By offering performance and reliability equivalent to today’s SCSI disk drives, SAS will have a wide repertoire ranging from application in workstations, servers to external storage. It could affect the Fibre Channel space in the future (when the price-dynamics are considered), but simultaneously offer convergence with Fibre Channel.

Two to tango

Essentially, the SAS interface was developed to leverage a common electrical and physical connection with SATA, enabling the SAS architecture to accept both SAS and SATA drives in a single enclosure. With this capability, a SAS array can provide a tiered storage environment in a single system. Serial SCSI offers vendors a terrific opportunity to take advantage of these ‘commodity’ parts to bundle innovative technology.

SAS and serial ATA devices use identical command sets (SATA uses a subset of the SCSI commands) and both types of devices can plug into the SAS connector. This means that when vendors design an architecture for SAS devices, they are at the same time designing an environment that will also work with SATA. This will result in savings considering the fact that the same design could be used for twin purposes. For example, a design that can be implemented in products that address high-performance (SAS) and low-priced (SATA) market segments.

SAS-SATA compatibility results from some cooperation between the SAS and ATA communities, and from the SATA group’s adoption of the well-proven SCSI command set.

Reliability

SAS drives are fast, robust, and identical to high-performance devices used in large enterprises. Not only are they built to specifications similar to the current crop of parallel SCSI and Fibre Channel drives but also make use of the same SCSI command set. In, addition it offers a convergence path with Fibre Channel in the future.

As the volume of data increases, storage needs seem to increase proportionately. Also, IT managers are constantly finding ways to store data in a cost-effective manner. While IT managers are still figuring out how they can allocate their information based upon application requirements, systems based on SAS could provide a new level of flexibility that will allow a single system to meet a wide range of requirements. Ultimately, if it reduces complexity of storage management and adds value to their investments, enterprises would surely feel better about things in store!

venkatesh@expresscomputeronline.com