Serial attached SCSI and serial ATA seek their levels.

Serial Attached SCSI (SAS) and Serial ATA (SATA) are two important technologies emerging in the market after years of positioning. and are seemingly poised for market success. We appear to be at a point in time where the cards are falling into place for both I/O technologies. This article will explore the capabilities of SAS and SATA, first as complementary interconnect schemes and secondly as high-volume storage devices. The article will also attempt to cut through some of the rhetoric surrounding both interfaces and draw some honest conclusions about where and how these important technologies will coexist within the mainstream storage markets.

Serial ATA: The Interconnect

Serial ATA emerged from a growing need to supplant the parallel ATA interconnects scheme within the high volume desktop and mobile client market. This mainstream market was being challenged by the need to reduce form factors, improve system airflow, reduce pin counts and move to lower voltage I/O signaling schemes. The physics of ever-faster processors, growing power budgets, and diminishing semiconductor feature sizes were dictating the changes.

[FIGURE 1 OMITTED]

While attempting to address these challenges, cost was always paramount. Gross margins in these very competitive, high-volume spaces of the market are ridiculously low and demanded a "no-frills" approach. These markets are primarily single tasking and driven by the need to serve the lowest common denominator in order to provide the most competitive pricing.

To facilitate the market migration from Parallel ATA toward Serial ATA, only a small premium, if any, could be demanded. The SATA interface was designed to be inherently half-duplex; it has no native capability for address expansion (except by adding additional ports), and has no command facilities for serving more than one host (multi-initiator), an essential element in servers. Other enhancements like command queuing and blinking LEDs are optional components of the architecture, not required in the most cost-sensitive of applications. While it was designed primarily to operate as a point-to-point cabling scheme within an enclosure, it is being adapted to backplane environments, as well as in some limited out-of-box cabling schemes.

Serial Attached SCSI: The Interconnect

The standard high-volume server market consumes nearly 80% of enterprise-class storage devices. In recent years, OEMs began looking for a technology that would ultimately replace Ultra320 SCSI solutions. The options were to continue with a next-generation parallel SCSI solution or to look at serial alternatives. Parallel SCSI was becoming increasingly constrained by the difficult physics associated with distributed transmission line buses, yet OEMs wanted to preserve the more than 20 years of SCSI "middle-ware" applications essential to business continuance in a large number of small-to-medium businesses. Legacy SCSI also suffered from large form factor connectors, in addition to the historical addressing limitations of 16 devices per bus. Neither of these limitations was particularly suitable for the emerging 2,5-inch form factor drives.

Of additional concern was the need to offer customers the option of high-capacity, low-cost Serial ATA drives for some portion of their customers, while supplying enterprise-capable drives for those more concerned with I/O performance and high availability. It was not very attractive for the OEMs to meet this customer need through multiple product offerings.

Serial Attached SCSI was developed to address these market demands, and is viewed as the next logical step in the evolution of SCSI. In the simplest of terms, SAS can be thought of as a faster version of the tried and true SCSI interface, with the following enhancements:

* Dual-porting for improved fail-over capabilities or to enhance performance

* Support for wide ports, which is the ability to incrementally add bandwidth by ganging multiple ports together

* Enhanced physical addressing for expansion to over 16,000 devices

* Support for native SATA compatibility at both the logical and physical connection levels.

Another way to think of SAS is as a functional superset of the widely touted SATA connection scheme, which embraces additional enterprise attributes. The SAS interconnection scheme readily accommodates either SAS or SATA storage devices, and allows OEMs and end users an unprecedented choice, heretofore unheard of in the storage market. Because SATA drives can operate over SAS connections, it's important for the market to carefully distinguish between these interfaces as connection schemes or these interfaces within the context of a storage device.

Storage Devices

With the complex demands being placed on data (demands that vary throughout the data's life cycle), it's doubtful that a single disk drive can ever service the entire range of market needs. It seems the market will always be bifurcated by its demand for drives that both target the enterprise's need for performance and high availability, and desktop-class deployments that emphasize cost and capacity.

SATA: High-Volume/Low-Cost Disk Drives

At the heart of the SATA value proposition is cost. Lots of storage at very competitive, high-volume price points. The SATA interface mirrors the capabilities of these "no frills" storage devices. Typical SATA storage devices have less complex circuitry (uni-processor architectures), less exotic head-drive assemblies, slower rotational and seek time speeds, less rigid housings (more rotational vibration), less firmware complexity (lower R & D) and a mean-time-between-failure (MTBF) rate based upon an assumed "8-to-5" workload, when compared with enterprise class drives. The cost of these drives are not guided by the interface cost as much as it is by the construction of the drives, and by the competitive nature of the high-volume desktop market.

As the digital world unfolds, data creation and accumulation are growing at unprecedented rates. That, coupled with new regulatory pressures to retain and preserve data, is driving the market to take advantage of these low-cost, high-capacity drives, especially in applications that have historically been reserved for tape. Nearline storage, disk-to-disk duplication, reference data, and data back up and restore applications are a few of these applications.

SAS: Volume Enterprise Storage

SCSI drives have served the mainstream of the enterprise market for over two decades. These drives are more focused on delivering continuous performance, under the most demanding of workloads. Fast servo controllers, dual-processor architectures, rapid seek times, low-rational latencies, rigid drive construction designed to minimize rotational vibration between adjacent drives, and an emphasis on 24X7 availability are just a few of the distinctions from their desktop-class brethren.

Because of the price/capacity premiums, which can be as high as 40% when moving to smaller form factor drives, enterprise-class drives will lead the market toward the 2.5-inch form factors. These markets tend to emphasis I/O performance over capacity and require small diameter platters in order to control the power required to spin at 15K RPM. The need for increased airflow for cooling will also accelerate the adoption of 2.5-inch drives in mainstream high-volume servers.

Markets Served by SATA

SATA drive pricing makes these drives attractive in a variety of market segments. We expect that most desktop and mobile markets will migrate rapidly to SATA drives. Emerging markets, like consumer storage for personal video recorders and home media servers, will also see increasing usage of SATA drives.

With the advent of SAS as a mainstream server connection, we expect that many categories of servers will allow for SATA drives to be used as an option. It's believed that a large number of SATA drives will be attached through SAS-capable interconnects in a large number of server deployments. While "SATA over SAS" connections will be prevalent, some low-end server categories will have SATA-only connection capabilities.

SATA will also make its presence as a drive attach "across the wire" in networked storage applications. Fibre Channel or iSCSI bridges allow SATA to be used in secondary enterprise storage solutions. The growing emphasis on data recovery or accessing large volumes of reference data will make SATA more prevalent for nearline storage applications. As with the mainstream server market segments, SAS interconnects provide a means to support SATA drives in these networked storage applications, while offering the option of high-performance/ high-availability drives through a common infrastructure. Here again, "SATA over SAS" will be one way this market gets served.

SATA markets include:

* Volume desktop markets

* Mobile computing platforms

* Workstations

* Less demanding server environments

* Consumer applications like Personal Video Recorders (PVRs)

* Secondary enterprise storage: nearline, reference and rapid restore capabilities

* Archiving (alongside tape)

Markets Served by SAS

Serial Attached SCSI (SAS) is the first and only data storage connection scheme that resolves the conflicting data center needs of being both low cost/high capacity-oriented while also providing for high-performance/high availability storage. As such, SAS is an interconnect scheme that will become prevalent in mainstream standard high-volume servers. Never before has the market been able to allow the users to de-couple the interface decision from the drive purchase. This common infrastructure can be used to service a broad range of market needs, while also enabling IT resources to be dynamically repurposed to meet the changing demands on data throughout its lifecycle.

In addition to providing native Serial ATA compatibility, SAS is also welcomed as an industry-standard means to advance the otherwise limited Serial ATA architecture. Using SAS controllers and expanders, Serial ATA topologies can be scaled in applications that require greater degrees of addressability, and to support other enterprise capabilities such as failover.

Serial Attached SCSI simplifies the way complex storage needs are met, and creates new levels of efficiency that the market is just now beginning to explore. The following examples of SAS implementations illustrate the growing need for greater efficiencies in storage deployment, and explain why leading storage OEMs are committed to delivering systems that embrace Serial Attached SCSI.

Initial SAS deployments include (but are not limited to) the following:

* A simple SCSI replacement scheme in standard high-volume servers

* A means for lowering the cost of ILM by enabling users to intermix enterprise- and desktop-class drives, utilizing common cables, connectors, enclosures, power supplies and backplanes

* Efficiently servicing a mix of customers (some requiring SCSI drives, others demanding SATA drives) with a single product offering (servers, workstations, etc.)

* Scaling Direct-Attached Storage (DAS) environments that demand legacy SCSI middle-ware compatibility

* A means for delivering "more capable" and/or "more complex" Serial ATA topologies

* Standard high-volume servers re-purposed to become "building block" components of NAS or virtualization solutions

* New and innovative ways for deploying SAS will continue to emerge, promising to provide our ever-demanding customers with more capability for fewer dollars.

SAS and SATA: Conclusion

SAS and SATA interfaces are on the threshold of success and will become pervasive in mainstream computing applications. The market must be careful to distinguish between SAS and SATA as interconnection schemes or as markets for products, due to the ability to operate "SATA over SAS" connections, SAS and SATA can only be viewed as complementary market-driven solutions that allow for an unprecedented degree of customer choice. It's becoming increasingly obvious how the technologies are positioned and which interfaces will be preferred in which segments; but ultimately it's all about customer choice.

                   Desktop Class           Enterprise Class
                      Storage                   Storage

Rotational         10K. 15K RPM            5400, 7200 RPM
Speeds
Rotational         5-12 radians/sec/sec    >21 radians/sec/sec
Vibration
MTBF               600K hours @            1M hours @
                   8-to-5 Duty Cycle       24X7 Duty Cycle
Variable Sector    NO                      YES
Sizes
Indicators         NONE                    LED activity light
Seek Times         13 msec                 5.7 msec

Figure 2: Examples of disk drive attributes and differences

Harry Mason is director of industry marketing at LSI Logic (Milpitas. CA)

www.lsilogic.com