Daring to Win - The SAS revolution

Evolution occurs as technology moves forward, but a quiet revolution is taking place in storage architecture as OEM server manufacturers and system integrators are beginning to provide Serial Attached SCSI (SAS) solutions that will change the landscape of storage. Historically storage technology goes through a cycle of incremental evolution every 18 months, but with 20 years of SCSI technology now transforming into SAS, it is certainly not an exaggeration to call it a revolution rather than an evolution. Industry analysts anticipate SAS market penetration to grow rapidly now that volume production has commenced, and SAS is expected to account for 60% of the market by 2007.

The shift from parallel to serial attached SCSI provides customers with an aggressive roadmap that will evolve from data transfer speeds of 3Gb/s link today to 12Gb/s link, according to the plans of the SCSI Trade Association (STA). Also, links can be aggregated in set of 4 or 8, virtually delivering user bandwidth 4 or 8 times higher. The SAS technology specification also allows Serial ATA (SATA) hard disk drives (HDD) to be used alongside SAS HDDs in the same installation. This will give IT managers additional flexibility to select the most appropriate drive technology for both on-line (transactional, high-availability) and near-line (archival, low availability) usage. Both small form factor 2.5 and larger 3.5 SAS HDDs are being produced by manufacturers that include Fujitsu, Hitachi, Maxtor and Seagate.

SAS benefits
The benefits of SAS are numerous, but can be grouped into four primary categories: performance, flexibility, scalability and reliability. The SAS specification includes an initial 3Gb/s transmission rate as well as advance command queuing, multiple point-to-point routing, and fault tolerance designs. The point-to-point architecture enables simultaneous read and write activity over the same port. The dual port capability allows connections among multiple initiators, thus providing a failover path for increased reliability. SAS cabling is more compact than that of other standards, providing better airflow plus simplified hot plug connections.

The user can also obtain much greater flexibility with SAS architecture. The backplane design and interface protocol allow for either SAS and SATA HDDs to be used, or a mixture of both this benefits both integrators and users in terms of balancing cost, performance and mission critical applications. Virtually any hybrid storage system can be built with the assurance of interoperability. SAS is also backward compatible with SCSI software and middleware, lessening the need for training on the upgraded system, or for modifications to legacy software.

SCSI scalability meets a need that is becoming critical with constant changes in system requirements. SAS uses expander hardware as a switch, which simplifies configurations for larger external storage applications. These can therefore be scaled with a minimal effect on latency, allowing bandwidth to be preserved for increased workloads. In addition, the expander allows for a large number of topologies to well over 16K mixed SAS/SATA hard disk drives. With dual-ported SAS hard disk drives, high availability systems can be built. The use of SAS expanders, along with dual-port SAS HDDs, allows for redundant systems with a maximum tolerance to faults. Backwards compatibility with the SCSI feature set is also a crucial feature for the demanding enterprise storage market.

Figure 1: SAS/SATA 3Gb/s universal connector architecture

Interoperability
Although SATA II 3 Gb/s is itself only just becoming established, its interoperability with SAS means that the latter is likely to become the de facto standard for architecture, allowing the flexibility for either type of drive, or a mixture of both, to be utilised. And an aggressive approach to pricing will mean that the SAS option is no more costly than SATA is today if it used in conjunction with SATA drive, but can scale if higher performance SAS disks are used.

SAS architecture essentially has two major advantages over SATA II 3 Gb/s. SAS is a full duplex (meaning that each port can, at the same time, perform both read and write operations) and supports up to two ports, compared with SATA II 3 Gb/s, which is only half-duplex (either read or write operations at any time but not both concurrently) and has a single port. The SATA II 3 Gb/s connector, although compatible with SAS, has only one side that is active whereas both sides of the SAS connector are active.

Both SAS and SATA will operate up to 3Gb/s, although SATA also has lower speed options that can be used in a mixed environment.

SAS architecture is actually designed for 3Gb/s and is a perfect fit for a mixed system running at that speed. Another option is to run SAS with SATA 1.5 Gb/s disks on the same controller ports, but the transfer rate of the link will be dropped to 1.5 Gb/s any time such a device is transferring data. In this instance, the 1.5Gb/s SATA drives must use their own dedicated SAS controller port rather than sharing a port expander.

SAS even begins to have an impact in the space traditionally occupied by Fibre Channel (FC), which normally operates at 2Gb/s, although 4Gb/s products have just come onto the market. As the ecostructures to further support SAS at a switch level evolves, SAS will penetrate the lower echelons of the FC enterprise market due to the fact that SAS HBA performance at 48Gb/s is far greater than FC HBAs - a 4G Dual port FC HBA supports 16Gb/s. The reliability of SAS is also equal to or better than that of FC, and it offers greater flexibility as a drive interconnect technology.

Physical distance will be one of the major factors in deciding which type of storage to install. SATA has a maximum cable length of 1m, which is fine when the drives are physically close but can be too restrictive for many applications. In contrast, SAS can permit cable lengths of up to 8m and SAS expanders optionally allow this segments to be extended by 8m each hop, while FC can operate at a cable length of 15m in copper, or up to 10km in optical fibre. Hence, FC will always be the answer for fibre-switch based solutions over a large-campus or multi-campus distributed storage system, but SAS will begin to fulfil many of the more compact enterprise requirements, such as a single-site IT centre using SAS drive technology.

While SCSI devices were limited to 15 on one storage adapter, SAS allows up to 252 devices to a single storage adapter using port expanders. If thats not enough scalability, then the theoretical limit of up to 16K devices provides a level of scalability unprecedented in storage. With the introduction of 500GB drives by Hitachi, even a 16-drive SAS architecture could offer 8TB of storage in total.

Architecting a SAS environment, therefore, will not cost any more than an equivalent SATA environment. Although the host bus adapters (HBA) that are currently on the market are more expensive than their SATA equivalents, this inequality will disappear when value-line SAS RAID solutions are introduced.

SAS products have been made available simultaneously to integrators, resellers, and whitebox manufacturers as well as to major server OEMs. Channel customers are already among the early adopters who are implementing SAS applications. One of the factors that have enabled early product development has been the collaboration between companies such as LSI Logic and Fujitsu among others in the development of open standards including the SAS specification itself. LSI Logic has also been assisting systems integrators by providing them with valuable sample hardware starter kits.

With both entry-level and midline SAS RAID controllers expected to have a similar price point to SATA controllers, it is reasonable to wonder why anyone would specify a SATA controller. The SAS architecture will provide a single future-proof solution to many different requirements, one that can readily be upgraded as the need arises. At an entry level the SAS controller can be populated only with SATA drives, at mid level a combination of SAS and SATA would be used, and at the top level a SAS-only solution would be used. The SAS drives will themselves be priced similarly to SCSI.

LSI Logic was the first to market in early 2005 with the SAS3442X HBA, and will again be the first when it launches a SAS RAID controller by the fourth quarter of 2005.

Table 1: SAS/SATA architecture and population

Market segmentation
The effect of low SATA drive prices has already changed the character of the market beyond all recognition. There is now a much higher demand for nearline storage, with SATA replacing tape drives because of their ability to re-build concurrently with putting data online. The demand for SCSI has remained constant, or even grown slightly, at the same time as a secondary market for SATA of almost the same size has developed alongside it. People have become accustomed to the security of being able to back up data at any moment of the day, and now have many more drives than before. In addition anything that is called a server is now by definition expected to have RAID included.

In the longer term, parallel SCSI will ship in tandem with SAS and will continue to be available for some time, but has already reached its peak and sales will diminish to virtually zero by 2010. The key question is now whether SATA will continue as a standalone product in RAID applications or whether it will become a subset of SAS architecture.

The main reason that SAS is becoming so competitive is that the availability of a MegaRAID SAS software product allows a controller to be priced at below $100 a price point that no SATA controller can better.

Roadmap
The timescales for the introduction of SAS will be rapid. High-end MegaRAID controllers are scheduled for introduction during September, with both midline controllers and value line controllers aimed at SATA applications following in September. Software-based SAS RAID controllers are also expected before the end of the year.

The cycle will be completed with the introduction of RAID 6, which offers even greater assurance of data security. Under RAID 5 if one disk fails it puts a strain on the remainder of the disks, and the failure of a second disk would cause irrevocable data loss. RAID 6 has two parity bits and can tolerate two concurrent data losses. Added to this is a much more robust and efficient architecture.

All the new SAS products will be RAID 6 compatible and firmware-upgradeable at no extra charge. Although some manufacturers will charge extra for this, LSI Logic is committed to providing upgrades free of charge.

Conclusion
SAS can in fact be thought of as SATA without limitations, offering a flexibility that has never before been available.

The clearly defined standards allow all companies planning SAS products to strive towards the same goals, with the result that the market will become very competitive and the customer will be the ultimate winner. SAS is also viewed as an interface that fulfils all requirements, from large computer installations down to small home office environments.

For resellers the potential offered by the SAS revolution can be enormous, provided they take advantage of the new technology as early as possible and understand how to use it to their benefit. They should ensure that they have registered for the free evaluation units to allow themselves to become familiar with the new techniques. The value-added SAS expert will have the capability to offer maximum flexibility at minimum cost SAS can truly be said to be returning the value-add concept to the reseller. In short, its a revolution that will benefit resellers, system integrators and end customers alike.

Joe Leader is the senior director of channel marketing and business development for LSI Logic storage adapters. He brings to the company 10 years experience in the storage and channel industry, and is an integral part of growing LSI Logics channel program and developing a world-class, global partner program. Leader is an advocate inside the storage adapter business unit and is a media spokesman for key channel marketplace and RAID storage adapter technology initiatives. Leader joined LSI Logic when it acquired American Megatrends in 2001.

Leader's educational background includes Master of Arts, Master of Business Administration and Master of Technology Management degrees from Emory University and the University of Phoenix respectively.