The benefits of Serial Attached SCSI (SAS)

Posted on July 01, 2005

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One key advantage is support for both SCSI and Serial ATA (SATA) disk drives.

By David Allen

The next generation of SCSI technology, Serial Attached SCSI (SAS), will bring more-flexible storage solutions to end users and storage/systems integrators in a variety of ways. Although SAS was not the first attempt to serialize the SCSI protocol, it is the first standard specification to provide an interconnect mechanism for both SCSI and Serial ATA (SATA). As such, SAS meets both enterprise and midrange/nearline storage requirements at relatively low costs, providing users and integrators with flexible storage architectures.

Unlike its parallel SCSI predecessors, the SAS protocol provides a tunneling mechanism for delivering SATA frames through SAS connection infrastructures, including physical cabling connections. These interchangeable drive and cable connectors ensure “plug and play” between both SAS and SATA targets (hard disk drives, DVD drives, tape, etc.). Similarly, protocol and connection compatibility allows OEMs to easily configure storage systems for multiple markets by simply altering disk drive options and characteristics. A single SAS enclosure or server can be configured to support enterprise and/or midrange/nearline applications. In addition, end users can easily configure or update systems by swapping high-performance SAS drives and low-cost SATA drives as appropriate.

Serialization of the SCSI interface overcomes the physical and functional limitations of parallel interface technology. Increased bandwidth requirements, as well as challenges presented by clock skew and power consumption, prevented SCSI from moving beyond the Ultra320 specification. Serialization addresses the parallel interface limitations while significantly reducing power consumption. SAS leverages technologies prevalent in other serial interfaces, including SATA, Gigabit Ethernet, and Fibre Channel.

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Initial SAS systems will support a 3Gbps data rate for SAS drives and will be compatible with SATA I (1.5Gbps) data rates through an idle interleave process. Bandwidth scalability is addressed in SAS via connection aggregation. In this scheme, each SAS connection between common endpoints acts as a logically bonded channel, and bandwidth increases as individual channels are combined. These aggregated connections are referred to as “wide ports” with common connection counts of 2 and 4. For example, a “x4” wide port aggregates four independent physical links and provides up to 12MBps of bandwidth.

Similarly, port aggregation provides for physical connection fail-over since each SAS connection acts independently. If a single connection in a wide port is lost, only the bandwidth is compromised, not the entire connection or session service. This feature is unique to SAS and is key to providing scalable storage systems.

The first SAS deployments are targeted at servers with direct-attached storage (DAS). In this model, SAS hard drives and other targets are attached to the host via one of two methods. The simpler method connects drives directly via point-to-point connections to the host controller without expansion devices. In this configuration, the SAS host controller is usually on the motherboard and provides varying levels of RAID for data protection (see figure).

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In the DAS model, scalability and physical connections are limited to the number of physical interface ports on the embedded SAS controller. Today, controllers typically provide either four or eight connections to hard drives or other targets. Multiple drive types (SAS and SATA) can be intermixed to suit application and performance requirements.

A second model allows for more-flexible and scalable designs that take full advantage of system bandwidth. In this architecture, a SAS expander provides modular connectivity (in groups or cascades) for both SAS and/or SATA targets. This “expanded” model provides for significant flexibility in server and storage architectures.

External SAS storage topologies vary with OEM and end-user requirements but one key theme exists: External enclosure designs must be based on architectures that support both SATA and SAS drives. As such, the same enclosure can be targeted at two distinct markets while reducing design skews.

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Fail-over is achieved via the connection scheme and the use of dual-port drives (see figure, above). While SAS drives are natively dual ported, SATA drives require a dual-port multiplexer.

External SAS storage systems are typically based on one of two models. One model uses a daisy-chain topology while the other leverages a star or switched approach. The expandable daisy-chain model cascades external SAS or SATA JBODs. This model is the more cost-effective; however, latency increases as the number of JBODs or other targets increase. The higher-performance model uses a switched architecture with a “fan-out” expander to provide for a highly scalable architecture with no increase in latency as JBODs are added. Both allow for customized storage systems that can provide enterprise level, high-transaction services with SAS drives or nearline storage with low-cost SATA drives.

Additionally, SAS architectures will address large storage capacities in relatively dense enclosures by taking advantage of the 2.5-inch (SSF) drive form factor. Increasing spindle counts increases performance in striped RAID configurations across either SAS or SATA drives.

SAS addresses bandwidth requirements not only by its serial architecture, but also through aggregation schemes that logically bind multiple connections together. This model provides multiple choices for system OEMs and integrators when tradeoffs among power, pin count, and bandwidth are required.

Features found today in enterprise-level SANs are creeping into SAS components. One example is comprehensive diagnostics. As SAS architectures mature, more-complete sets of diagnostics and tools will be required for debugging of subsystems during development and also for installed production equipment.

SAS expanders enable storage network fabrics. SAS architectures support numerous storage targets as well as multiple hosts. In a fabric architecture, access control techniques become critical. SAN features such as zoning are now being addressed by standards organizations. Zoning will be particularly important for disk-less blade environments, where each blade has its own protected boot drive in an external enclosure. Future enhancements to SAS will likely include encryption and virtualization.

David Allen is on the board of directors of the SCSI Trade Association and director of strategic marketing at Vitesse Semiconductor.


HP embraces SAS

Among the major server/storage vendors, Hewlett-Packard appears to be ahead of the pack in its support for the emerging Serial Attached SCSI (SAS) disk interface. HP will begin to phase in support for SAS across its entire line of ProLiant servers this summer.

HP’s “standard” SAS drives are 2.5-inch, 10,000rpm small-form-factor (SFF) units in capacities of 36GB or 73GB, but the company will also support 3.5-inch, 15,000rpm, 147GB versions in a small percentage of ProLiant servers and in some external Modular Smart Array (MSA) disk arrays. HP will continue to support Ultra-320 SCSI drives for approximately one year, according to Rich Palmer, director of product marketing at HP. Support for 2.5-inch, 15,000rpm drives is expected around the end of 2006.

HP is partnering with Hitachi Global Storage Technologies (Hitachi GST), Fujitsu, and Seagate as drive suppliers.

Palmer notes that the 2.5-inch SFF drives will enable HP to pack 2x more drives in the same form factor for increased performance, as well as lower power consumption. For example, a 1U server can be configured with four SSF SAS drives. Performance figures for SAS-equipped ProLiant servers were not available at press time. SAS-equipped ProLiant servers will cost approximately the same as servers configured with Ultra320 SCSI drives, according to Palmer.

In other SAS-related product news, LSI Logic is shipping a wide variety of SAS components. Most recently, the company began sampling a SAS controller that supports PCI Express and 1.5Gbps or 3Gbps SAS, as well as SATA. The 4-port LSISAS1064E and 8-port LSISAS1068E RAID controllers both support eight PCI Express lanes at 2.5Gbps, for an aggregate bandwidth of up to 2GBps, or 4GBps full duplex. The controllers can be configured as x8, x4, or x1 PCI Express devices.

LSI is also shipping to OEMs (including Hewlett-Packard) a SAS RAID-On-Chip (ROC)-the LSISAS1078-and has demonstrated the chip with RAID 6, which protects against two drive failures. LSI is also shipping SAS host bus adapters (HBAs) and expanders, including the 12-port LSISASx12.

Rancho Technology recently began shipments of two 1U, rack-mounted, 3Gbps, 12-port SAS expanders. Introduced at last month’s Storage World Conference (SWC), the edge and fanout expanders allow users and/or integrators to increase the number of SAS or SATA devices and provide fault-tolerant path redundancy. Also at SWC, Rancho introduced 2.5-inch JBOD disk enclosures that support SAS, SATA, and Ultra320 SCSI. - DS


SAS progress report

Serial Attached SCSI (SAS) components (chips, controllers, etc.) entered the market late last year, and servers, blade servers, and subsystems for end users are expected in the second half of this year.

The SCSI Trade Association (STA-www.scsita.org), in conjunction with the University of New Hampshire, has conducted four SAS “plugfests” so far. The most recent one was in April, and the next one is slated for late September.

The plugfests are designed primarily to test interoperability of products from different vendors. Twenty vendors participated in the April event, which featured a demo configuration with 110 disk drives and interoperable disk drives (SAS and Serial ATA, or SATA), controllers, host bus adapters (HBAs), expanders, enclosures, and test equipment.

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Harry Mason, president of STA and director of strategic alliances for the storage components division of LSI Logic, says that most of the activity at the next plugfest will focus on exception conditions, error injections, corner cases, etc.

At last month’s Storage World Conference, STA hosted a number of SAS demonstrations, including an interoperability demo with 8-port PCI-SAS HBAs, hardware-based RAID (including RAID 6), a 12-drive SAS/SATA external enclosure, and four-drive internal enclosure with 15,000rpm, 36GB, 3Gbps SAS drives, and SATA drives.

Dual-ported SAS disk drives are available in 2.5-inch and 3.5-inch 10,000rpm models, as well as 3.5-inch 15,000rpm models. - DS


Hitachi ships SAS, 4Gbps FC drives

By Ann Silverthorn
This month, Hitachi Global Storage Technologies (Hitachi GST) began shipping 15,000rpm Ultrastar 15K147 hard disk drives with either Serial Attached SCSI (SAS) or 4Gbps Fibre Channel interfaces. The company claims to be first to market with 4Gbps Fibre Channel drives.

“SAS will replace parallel SCSI,” says Sam Sawyer, senior advisor, product planning and strategy, at Hitachi GST, “and SAS [subsystems] will accommodate both SAS drives and SATA [Serial ATA] drives, so you can configure a storage system with very fast drives for online transaction processing types of applications as well as lower-cost ‘bulk storage’ in the same enclosure.”

Sawyer points out that SAS offers much greater connectivity than parallel SCSI. “You can connect 15 drives on a [shared] parallel SCSI bus, but a SAS infrastructure will accommodate more than 16,000 connections between drives, HBAs, and expanders. So, you can get much more complicated configurations, such as SANs, but in a local area.”

SAS supports data-transfer rates of up to 3Gbps.

The 4Gbps Fibre Channel drives provide a data-transfer rate of up to 400MBps half-duplex and up to 800MBps full-duplex per port.

The new drives are available in 36GB, 73GB, and 147GB capacities on two, three, or five platters and feature 16MB caches. Hitachi GST will continue to offer Ultrastar 15K147 drives with Ultra320 parallel SCSI interfaces.

Other drive specs include an average seek time of 3.3ms to 3.7ms, average latency of 2ms, and 33% more I/Os per second than Hitachi’s 10,000rpm drives.


Why SAS? A little history

About four years ago, Fibre Channel was healthy, Serial ATA (SATA) was in development and showed promise, and the venerable parallel SCSI interface looked like it would end its run at the Ultra320 specification. However, a group of vendors were looking for an interface that would provide an alternative to, yet pull strengths from, both Fibre Channel and SATA.

“We considered ‘dumbing down’ Fibre Channel [to achieve lower costs], ‘smartening up’ SATA [for better performance and reliability], and another path [which eventually became the Serial Attached SCSI, or SAS, specification],” explains Harry Mason, president of the SCSI Trade Association (STA) and director of strategic alliances in the storage components division of LSI Logic.

Mason says the group had a number of objectives, including preserving 20 years of SCSI software compatibility; plug compatibility with SATA; better scalability than parallel SCSI; and price equivalence with parallel SCSI.

“SATA didn’t fit [some] of those objectives because to grow it into a multi-initiator model would have required new commands, which we already had with SCSI,” says Mason. “Fibre Channel at the time was not oriented to our cost objectives…and it couldn’t support SATA.” As such, the group began work on a device-level interconnect that would provide a successor to parallel SCSI for server-storage interconnection while also providing plug-compatibility with SATA drives. For more information, visit www.scsita.org. - DS


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