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Serial-Attached SCSI: the universal enterprise storage connection. (Connectivity).

The move to serially connected storage devices, primarily disk drives, is irrefutable. While today's installed base is relatively small and consists of a few million Fibre Channel drives, new interfaces that service a greater percentage of the storage market promises to change all of that. The number of serially connected storage devices deployed over the next four years could be well in excess of 100 million drives!

VLSI integration and the advances of high-speed serial transceiver technology are rapidly driving the industry toward these versatile connection schemes. The advantages of these serial connections are many; smaller form factors, more flexible and thinner cabling, less weight, more predictable/reliable signaling mechanisms, more deterministic fault isolation and topologies that promise to scale with the needs of the end-users.

Serial ATA

Serial ATA (SATA) is an emerging storage connection that promises to change the way the world connects its storage devices. SATA is expected to be a replacement for parallel ATA and, as such, will be focused on bringing the best "cost-pergigabyte" drives to the market. With market volumes in the desktop PC space exceeding 150 million units per year, cost will be the driving force behind the manufacturing of these devices.

The serial connection scheme used to interconnect these drives within the host is much more flexible and does less to restrict the airflow inside of desktop enclosures. Because of this, OEMs have the ability to support a wide variety of systems with essentially a common drive connection cable, something rarely achievable with the parallel ATA implementations. With integrated circuit providers, system manufacturers and drive suppliers focused on delivering SATA, it promises to make significant market in-roads over the next two years.

In addition to the desktop market, SATA is also finding reception in the emerging market for near-line storage systems. Enterprises struggle to provide cost-effective storage solutions to support high performance backup, infrequently accessed data, or write-once/read-mostly types of data. Disk drives have become increasingly more competitive with other back-up storage technologies such as tape, and have proven to be faster when backing up the huge amounts of data required in today's data-center applications. SATA promises to play an important role in these new segments of the market, due to its low-cost-permegabyte focus.

The Changing Data Center

The data center has a variety of storage requirements. Some applications are more focused on transaction processing performance, while others are more concerned with low-cost, high availability or system scalability. Cost-per-gigabyte and cost-per-I/O, balanced by various degrees of data availability and scalability have historically dictated substantially different storage solutions, depending upon the user's primary application needs.

SCSI has been the dominant enterprise storage interface for much of the past two decades. While SCSI will continue to service standard high-volume servers and storage subsystems over the coming decade, it's clear that evolving enterprises will require more from this venerable storage interface.

Serial-Attached SCSI

Serial-Attached SCSI is the latest rebirth of SCSI and promises to greatly expand upon the existing capabilities of this enterprise storage connect. Serial-Attached SCSI intends to leverage the 20 years of proven SCSI legacy middleware/software that is entrenched in the enterprise, as well as deliver a powerful small form factor connection scheme, capable of achieving substantially more functionality than we have seen from SCSI heretofore.

Serial-Attached SCSI offers a full, dual-ported connection that supports the most stringent of high-availability requirements. In addition, Serial-Attached SCSI will extend drive addressing up to 4,032 devices per port, well beyond SCSI's current limit of 15. As with SATA, Serial-Attached SCSI is a high-speed, point-to-point technology initially designed to operate at speeds up to 3Gbps.

System designers have found that point-to-point serial connections are inherently more reliable than shared bandwidth serial connections. As a result, point-to-point serial connections have become the preferred method for implementing high-availability systems. Historically, the scaling of serial device connections was accomplished by using a shared loop topology. While this is an effective way to scale, it does offer some challenges in high availability systems in terms of requiring high-speed shared bandwidth connections at each device connected on the loop. It also creates challenges for providing effective means fault isolation and simplifying the process for initializing the loop.

In order to accomplish the scalability required for large storage systems using point-to-point connections, the ability to cost-effectively integrate multiple-ported serial connections on a single VLSI device is required. These VLSI advances enable a new generation of integrated circuits that provide multiple point-to-point drive connections within a single device.

Serial-Attached SCSI takes advantage of these VLSI advances to enable a highly scalable connection scheme and to allow the connection topology to grow through the use of expanders. Expanders provide the necessary fan-out to create large enterprise-worthy configurations while maintaining legacy compatibility with today's proven SCSI software.

Perhaps the most significant of all the additions being offered by Serial-Attached SCSI is the ability to provide customer choice. Developers wanted to offer developers and users at all points on the value chain--OEMs, VARs, SIs and end-users--unprecedented choice and flexibility in configuring their enterprise systems. For example, for the first time, OEMs will be able to populate boxes with drives according to their customer's choice without changing the system itself. By enabling the ability to deploy a diverse range of system capabilities through a common storage connection interface, Serial-Attached SCSI promises to dramatically change the way we think about serving the storage market.

As we have seen in the networking world, standards like Ethernet are capable of supporting a wide variety of connection-oriented protocols. Serial-Attached SCSI was architected at its inception to support three separate protocols. The Serial SCSI Protocol (SSP) is used to leverage the volumes of existing SCSI software. The Serial Management Protocol (SMP) is used simply for managing the new serial point-to-point topology, while the SATA Tunneling Protocol (STP) allows Serial-Attached SCSI to interface seamlessly with SATA devices. This multi-protocol support, coupled with SATA-compatible signaling and a physical mating scheme capable of directly connecting SATA drives, offers a powerful construct for deploying storage systems that deliver a wide range of storage value.

The Serial-Attached SCSI connector is form-factor compatible with SATA. SATA drives will plug directly into Serial-Attached SCSI connectors and if supported in the system, will be able to perform as expected. The SSP protocol has the facility to query the device and determine if the drive is a Serial-Attached SCSI drive or a SATA storage device and respond accordingly. On the other hand, Serial-Attached SCSI drives are keyed to prevent them from being inappropriately plugged into a SATA connection.

It's expected that many of the early Serial-Attached SCSI disk drives will support dual porting. By taking advantage of the space between the SATA power connector and the SATA data connector, Serial-Attached SCSI cleverly maintains form-factor compatibility with SATA, while allowing for the addition of the second port. This dual-ported functionality is an important aspect in making Serial-Attached SCSI robust enough to span a broad range of applications.

The SCSI market has traditionally been within servers (in the box), close to the server (near-box) or deployed within externally connected NAS and SAN storage subsystems. By offering Serial-Attached SCSI, which preserves legacy SCSI software, existing SCSI users will have a viable migration path to the future. In addition, the capability of supporting SATA drives through a common connection scheme, allows systems to be deployed that can use either Serial-Attached SCSI drives, for their mission-critical applications that require high performance and high-availability characteristics, or SATA drives that will provide a low-cost-per-megabyte alternative in near-line applications.

Systems implemented using this universal connection scheme can leverage common backplanes and common connector/cabling schemes, which span the range of enterprise storage system needs. Serial-Attached SCSI enables a new breed of storage systems by consolidating the infrastructure required to deliver the functionality, compatibility and capability to make it truly a universal connection for the enterprise. Serial-Attached SCSI is well into the standardization process and systems are expected to be shipping in 2004.

www.t10.com

www.scsita.org

Harry Mason is director of industry marketing at LSI Logic (Milpitas, Calif.) and president of the SCSI Trade Association
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Article Details
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Author:Mason, Harry
Publication:Computer Technology Review
Geographic Code:1USA
Date:Dec 1, 2002
Words:1351
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