SAS: The new kid on the I/O block

Posted on January 30, 2007

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With the recent addition of Serial Attached SCSI (SAS), end users and systems/storage integrators have a wide array of choices in enterprise-level disk drives and host connections.

Dave Simpson

Take your pick: Fibre Channel, Serial ATA (SATA), or Serial Attached SCSI (SAS). And don't forget 20-year-old parallel SCSI. Serial technologies such as Fibre Channel, SATA, and SAS are rapidly taking over the enterprise disk array market, and SAS is the new kid on the I/O block. But before we delve into SAS, bear in mind that "old" interfaces still account for the vast majority of disk drive shipments.

Gartner Dataquest divides the hard disk drive market into multi-user and single-user segments, with multi-user disk drives approximately equating to what other research firms refer to as enterprise (as opposed to desktop) disk drives. And according to Gartner’s estimates, good-old parallel SCSI accounted for 42.8% of all multi-user drive shipments last year (most of which were Ultra320 SCSI drives, which is the current-and last-generation of parallel SCSI). SATA accounted for almost one-fourth (24.7%) of multi-user drive shipments, while Fibre Channel accounted for about the same-24.1%. SAS’s market share was still in the single digits last year (see figure).

But looking forward, the trend toward serial interfaces is clear. By the end of next year, parallel SCSI is expected to account for a mere 7.3% of multi-user drive shipments, while SAS will garner about a 41% share, according to Gartner. Fibre Channel and SATA are expected to basically maintain their market shares, at 22.9% and 28.6%, respectively. If those predictions hold up, it’s clear that SAS’s rise in the market will be almost 100% at the expense of its predecessor-parallel SCSI-with little effect on the market shares of high-performance Fibre Channel drives and low-cost, high-capacity (up to 750GB) SATA drives.

John Monroe, research vice president with Gartner Dataquest Research, summarizes: “Parallel ATA dominates in all entry-level server markets. Serial ATA will continue to dominate in low-end servers and nearline storage applications. SCSI/SAS will dominate in legacy DAS [direct-attached storage] markets and in midrange FAS [fabric-attached storage] systems. And Fibre Channel will dominate in some midrange and most high-end FAS systems.”

Although differing from Gartner’s predictions, disk-drive shipment projections from International Data Corp. (IDC) paint a similar overall picture, although IDC’s figure are less optimistic regarding SAS’s market penetration. IDC separates the market into “enterprise” and “desktop” segments.

For example, IDC estimates that SAS hard drive shipments accounted for about 7% of the enterprise segment last year and will account for 23.2% by the end of next year. In that same time frame, IDC expects SATA to have an almost 40% market share, followed by 24.3% for Fibre Channel and 12.7% for parallel SCSI (see figure on p. 28). John Rydning, research manager for hard disk drives and components at IDC, expects SAS to account for about 26% of all enterprise-level drive shipments by 2010.

Analysts note that the integration of Intel’s SAS chip sets on motherboards will spur adoption of SAS, as will support from the top-tier server suppliers. Hewlett-Packard, IBM, and Dell are among the top-tier server vendors that are already shipping SAS-based servers.

Unlike parallel transmission technology, which transmits data in multiple streams, serial technology sends data in a single stream. As such, it’s not tied to a particular clock speed and can send data much faster. SAS overcomes the limitations of parallel SCSI, such as signal skew and crosstalk, signal termination restrictions, cable and connector reflection, lack of scalability and performance, and device addressability (128 devices with SAS, or more than 16,000 devices per domain with expanders, compared to 15 with parallel SCSI).

Current-generation SAS drives run at a maximum of 3Gbps, or about 300MBps. Ultra320 SCSI is spec’d at 320MBps. (The latest generation of FC drives run at 4Gbps, or 400MBps.) But unlike parallel SCSI, SAS enables full-duplex, bidirectional data transfers, with speeds of 3Gbps in both directions, effectively doubling performance.

In addition, parallel SCSI uses a shared-bus architecture in which all connected devices share a common bus, which limits performance as more devices are added. In contrast, SAS (like all serial technologies) uses a point-to-point connection scheme to maximize throughput. In point-to-point connections, each device has a dedicated path to the controller, with full bandwidth on each connection.

SAS also features multi-lane connections so that, for example, an “x4” (or “4x”) connection provides up to 24Gbps aggregate throughput. Of course, all performance specs refer to theoretical maximums. Your results will vary.

Quantifying performance differences among SCSI, SAS, SATA, and Fibre Channel at the subsystem level is impossible because performance depends on a wide variety of variables, such as controller implementation, number and size of drives, drive rotation rate (rpm), type of I/O workload (small or large block transfers), etc.

Although all-SAS arrays could perform better than pure Fibre Channel arrays, Fibre Channel arrays are still considered to be the fastest. However, the arrival later this year of 15,000rpm, 2.5-inch (small-form-factor) SAS drives will significantly boost the performance of SAS systems. Seagate, for example, expects to hit production shipments of 2.5-inch, 15,000rpm SAS drives in the fourth quarter of this year, according to Willis Whittington, a product marketing manager at Seagate.

Currently shipping 2.5-inch SAS drives rotate at 10,000rpm. Today’s 3.5-inch SAS drives rotate at 15,000rpm. That compares to 7,200rpm for the majority of SATA drives shipping today, although 10,000rpm SATA drives are ramping up. Like SAS, Fibre Channel drives are available in 10,000rpm and 15,000rpm versions (see table for interface comparisons).

“Although it’s generally true that Fibre Channel systems will provide higher performance than SAS systems, it’s too early to say how much more performance,” says Seagate’s Whittington. “So far, the performance differences have been in the single-digit percentage point range.”

Although agreeing that it’s too early for definitive comparisons of all-SAS arrays versus all-Fibre Channel arrays, Gene Lee, vice president and CTO in the Xtore division of AIC, predicts that SAS systems will provide significantly faster performance because of their multi-lane connections-even compared to 4Gbps Fibre Channel.

Comparing SAS and SATA at the drive level, Xtore’s internal testing produced 92MBps to 95MBps in sequential reads with SAS drives vs. 60MBps to 70MBps for SATA drives.

Like Fibre Channel drives but unlike parallel SCSI and SATA drives, SAS drives are natively dual-ported (two channels per drive), enabling redundant drive connections and better reliability and performance.

For systems/storage integrators, the key advantages of SAS over parallel SCSI may be related to packaging issues. For example, the cables for SAS are simpler and thinner and have far fewer (7 vs. 68) pins than parallel SCSI ribbon cables. Thinner cables help OEMs solve cooling and airflow issues and also facilitate the integration of 2.5-inch drives into traditional 1U and 2U servers and blade servers.

SAS cable length is limited to 8m per connection between devices (e.g., drives, controllers, expanders), although total domain cabling can theoretically scale to thousands of feet. Parallel SCSI is limited to 12m for the entire connection.

SAS also introduces expanders as an option for adding capacity (see figure on p. 28). Expanders are basically simplified switches that connect initiators and targets to create a SAS “domain” with direct, table, or subtractive routing. SAS expanders can connect hosts, hard drives, or other expanders.

SAS also introduces expanders as an option for adding capacity (see figure on p. 28). Expanders are basically simplified switches that connect initiators and targets to create a SAS “domain” with direct, table, or subtractive routing. SAS expanders can connect hosts, hard drives, or other expanders.

There are two types of SAS expanders: edge and fan-out. An edge expander allows for communication with up to 128 SAS addresses, allowing a SAS initiator to communicate with these additional devices. A fan-out expander can connect up to 128 sets of edge expanders, allowing for even more SAS devices to be addressed.

Other performance-related benefits of SAS include “wide” (combined) ports for aggregated bandwidth, full-duplex operation, and advanced command queuing (called tagged command queuing).

SAS is often said to be more reliable than SATA and about on par with Fibre Channel. However, reliability is a function primarily of drive construction and is not related to the interfaces or protocols.

Seagate, for example, specs both its FC and SAS drives at a mean time between failure (MTBF) rating of 1.4 to 1.6 million hours, depending on drive model. In comparison, Seagate specs its “enterprise SATA” disk drives at 1 to 1.2 million hours MTBF and its “desktop SATA” drives at 700,000 hours. However, MTBF ratings vary depending on drive vendor.

In terms of cost, SAS drives today cost about as much as Fibre Channel when equalized on a per-GB basis, although they’re much more expensive than SATA drives on a per-GB basis. As SAS drives ramp up, analysts expect the price differential between Fibre Channel and SAS to approximate today’s price differences between parallel SCSI and FC drives.

Despite all of the above-mentioned benefits, the one hyped most by vendors is the fact that SAS enclosures can support both SAS and SATA drives, enabling in-the-box tiered storage. (It’s important to note that the converse is not true: SATA enclosures cannot accommodate SAS drives. And one drawback to SAS is that it’s not backward-compatible with parallel SCSI drives.) However, it’s unclear whether users will actually want to intermix drive types in the same enclosure.

“Users tend to choose an interface type for a specific application or set of applications,” says Paul McLeod, technical sales/marketing manager at Infortrend. “Tiering different drive types in the same enclosure may be more to the advantage of systems vendors because they can use the same enclosure to support either drive type [as opposed to both drive types].”

McLeod notes that it will be much more common for end users to link all-SAS or all-SATA arrays behind the same SAS controller in a SAS-SATA infrastructure. In addition, users may want to use SAS drives as a sort of cache in front of a larger number of SATA drives.

“Arrays are typically deployed for one type of application, whether it’s high-speed transaction processing or for reference data,” says Paul Vogt, director of product marketing at Adaptec. “As a result, mixing drive types in the same enclosure will be more the exception than the rule. But the bottom line is that users and integrators have a choice.”

“If you need high capacity at low cost you’ll fill your system with SATA drives, but if you need high performance you’ll go with SAS,” says Steve Gardner, director of product marketing in LSI’s Engenio Storage Group. “Intermixing drive types will really come into play when you’re doing storage consolidation, where you have different servers with different requirements all sharing the same [SAS-SATA] storage system.”

Another area where intermixing drive types in the same enclosure may make sense is in separating data types. For example, Xtore reports that some early users of its 16-drive SAS arrays are configuring the systems with, say, four SAS drives dedicated to the operating system and RAID while using SATA for the remaining 12 drives and application data.

Another possible scenario is in digital content creation studios where editors could put their interactive content on high-speed SAS drives and their archived content on low-cost SATA drives-in the same enclosure.

Although vendors often refer to SAS and SATA as being complementary, they are also competitive. This has become particularly true with the advent of “SATA II” (aka “enterprise-SATA”) drives, which have improvements such as hot-swap capability, port multipliers that enable connections to as many as 15 drives (although fewer connections provide better performance), 2m cabling, and native command queuing (NCQ). In addition, SATA-II drives run at 3Gbps-the same as SAS drives-and come in capacities as high as 750GB, compared to 300GB for most SAS drives.

For both drive types, the next speed on the road map is 6Gbps (see figure on p. 31). In the end, however, users will make their drive interface decisions based on the same factors that they always have: cost, performance, and reliability, with reliability becoming less of a factor as the different drive types approach reliability parity.



HP DAS arrays support SAS, SATA
By Kevin Komiega
Hewlett-Packard expanded its line of entry-level storage systems for small and medium-sized businesses (SMBs) last month with the debut of two new direct-attached storage (DAS) disk arrays with support for multiple drive types and price tags of well under $5,000.

The new systems-the StorageWorks MSA60 and MSA70-support Serial Attached SCSI (SAS) disk drives, as well as low-cost, high-capacity Serial ATA (SATA) drives in the same enclosure.

The MSA60 supports 3.5-inch SAS or SATA drives for a maximum capacity of up to 3.6TB with 12 SAS drives, or 9TB with 12 SATA drives. It can be directly attached to HP ProLiant and Integrity servers. Cascading of enclosures in a 1+3 configuration allows users to daisy-chain the systems to increase capacity.

The MSA70 supports 2.5-inch small-form-factor (SFF) SAS or SATA drives for a maximum capacity of 3.6TB with 25 SAS drives or 1.5TB with 25 SATA drives. Users can attach the disks to ProLiant or Integrity servers and can cascade enclosures in a 1+1 configuration.

Both systems take up two rack units and offer data-protection features such as RAID 6 with Advanced Data Guarding to prevent downtime or data loss during upgrades or in the event of drive failures.

HP has recently been on a mission to migrate SMB customers to networked storage. So why roll out the direct-attached gear? HP says an estimated 60% of smaller companies are still living in a DAS world, and it would be a mistake not to provide new products to those customers.

“The ultimate goal is shared, net-worked storage,” says Charles Vallhonrat, product marketing manager for entry-level SANs in HP’s StorageWorks division. “Shared storage provides users with better total cost of ownership over time, but we still have to provide customers with the building blocks. We want to make sure there is an easy path for them to get to [networked storage].”

The StorageWorks MSA60 is available now, while the MSA70 is expected to be available “in the first half of 2007.”

The MSA60 is priced from $2,999. As for the MSA70, HP says the “average US list price will not exceed $3,250.”



A sampling of SAS products
Note: Most of the following products were announced in the fourth quarter of 2006, although not all of them are available yet.

Dell is shipping SAS-based PowerVault storage servers that support both file- and block-level I/O(see “Dell, Microsoft launch ‘unified storage’ array,” p. 8).

LSI’s Engenio division last month began shipments to its OEMs of SAS-SAS (host and drive connections) external RAID arrays. The company also introduced easier-to-use management software because the SAS arrays are designed primarily for small to medium-sized businesses (SMBs).

The model 1331 has one host port per controller and the model 1333 has three host connections per controller, enabling it to be used in clustered configurations (two- or three-node) with SAS-based shared storage.

The systems are based on a 2U, 12-drive enclosure. The 1333 has six 3Gbps SAS interfaces for host connectivity and two SAS interfaces for capacity expansion. OEMs and integrators can configure the RAID arrays in two-drive configurations with 146GB of capacity up to 48-drive configurations with 14TB of capacity using 300GB SAS drives. Each array is available in single- or dual-controller versions. Although SAS systems typically support both SAS and Serial ATA (SATA) disk drives, LSI’s arrays are currently qualified only with SAS drives. Support for SATA will come later this year.

Using the same code base as LSI’s SANtricity Storage Manager software, the new Simplicity Storage Manager software is an easier-to-use version. The company claims that with the software users can configure the storage systems in fewer than 10 clicks. Standard features of the Simplicity software include automated I/O path fail-over, online capacity scaling, and RAID group expansion. Optional features include storage partitioning, snapshots, and volume copies.

The company sells the SAS-SAS hardware and management software primarily through its OEMs, which include vendors such as IBM, SGI, and Sun.

SGI has already announced a RAID array based on LSI’s SAS systems. SGI’s InfiniteStorage 220 RAID system can be configured with either SAS or SATA drives, and with either Fibre Channel or SAS host connections. Using 500GB SATA drives, total capacity is 24TB per system. Shipments are expected within the next month or two, with entry-level pricing of about $8,000.

Adaptec’s HostRAID SAS 44300 and 58300 controllers are entry-level, low-profile devices with mini SAS connectors and software-based RAID (levels 1, 0, and 10). The HostRAID 58300 card has two x4 external connectors for attachment to JBOD or RAID subsystems.

Features of both cards include support for up to eight direct-attached SAS and/or SATA drives (or up to 128 drives using expander technology) and support for 64-bit/133MHz PCI-X host interfaces. Pricing starts at $250.

The cards are part of Adaptec’s Unified Serial Controller family, which is based on Intel IOP348 I/O processors, and supports RAID 6 and RAID 5EE (Adaptec’s performance-optimized RAID, also referred to as “hot space”), and SAS, SATA, or parallel SCSI drives.

Finisar’s Xgig Jammer SAN testing device supports SAS, Fibre Channel, Ethernet, and in the latest (2.6) version, SATA (1.5Gbps and 3Gbps). Xgig Jammer can be used to test protocol compliance and create “what-if” scenarios by allowing user-specified modification of actual network protocol traffic and to simulate error test cases to improve error checking. The tester supports SAS/SATA protocols such as SSP, STP, and SMP, as well as out-of-band signaling and spread spectrum clocking, and can be used with Finisar’s Xgig Analyzer.

Infortrend is shipping the S12F-R/G1420, a 2U 12-bay RAID controller that supports two 4Gbps Fibre Channel host connections and 12 SAS or SATA drives. The controller is available with up to 4GB of cache and is available in single- or dual-controller configurations. Tagged command queuing supports up to 256 commands. Infortrend has also announced SAS-SAS (host and drive connections) controllers.

The Xtore division of AIC is shipping a 4U, 48-bay SAS/SATA JBOD system, the XJ-SA24-448R-S, that stores up to 36TB. The enclosure comes with dual-channel SAS and single-channel SATA, with dual-channel SATA available as an option. Add-on modules include dual Fibre Channel host ports, dual SAS host ports, and dual iSCSI ports. The SAS/SATA backplane provides up to 48Gbps via x4 host/expansion channels, with two per interface module. All of the 3.5-inch drives are hot-swappable. The enclosure uses a “dual-drive brick-carrier” design that the company says can double the capacity density. The JBOD arrays are based on SAS/SATA controllers from Adaptec.

Due later in this quarter, Xtore’s XD-2000 series of SAS RAID enclosures include a technology the company refers to as “RAID 50 Interleave,” which provides protection against multiple drives failures (similar to RAID 6) but with improved performance over traditional RAID-5, RAID-50, or RAID-6 implementations, according to the company. Two models are available: The XD-S226-316-S is a 3Gbps SAS RAID implementation with dual controllers, six channels, and 16 bays in a 3U enclosure supporting up to 12TB and a throughput rate of 48Gbps. The XD-F224-316R-S includes 4Gbps Fibre Channel host interfaces, dual controllers with dual channels per controller, 16 bays in a 3U enclosure, and up to 12TB of capacity and a throughput rate of up to 8Gbps.

Arena Maxtronic’s 3U JanusRAID 6651/6652 disk subsystems are available with either two 4Gbps Fibre Channel host interfaces (model SS-6651S) or two 3Gbps SAS host interfaces (models SS-6652S and SS-6652J JBOD) and up to 16 3.5-inch SATA-II drives for a total capacity of 16TB. The single-controller arrays support all RAID levels (including RAID 6 and the company’s NRAID) and include dual flash ROM on each controller, AMCC’s PPC440SP controller CPU, and up to 1GB of DDR RAM. PathGuard software provides I/O path fail-over, failback, and load-balancing.

Promise Technology’s VTrak E-Class SAS storage systems include versions with Fibre Channel host connections (E310f) and SAS host connections (E310s). Both are 2U, 12-drive subsystems that support RAID 6. Features include dual 4Gbps Fibre Channel host ports per controller (E310f) or dual 3Gbps SAS host ports per controller (E310s), support for SAS and SATA drives, RAID 0/1/5/6/10/1E/50, dual active/active RAID controllers, and JBOD expansion support for up to 60 additional drives.

Xyratex has demonstrated its E5402E RAID array, which supports 3Gbps SAS and SATA drives and RAID 6. The 2U, 12-drive SAS-to-SAS (host and drive connections) subsystem is based on SAS components from LSI. Xyratex claims performance of more than 800MBps in a dual-controller configuration with sequential read operations, and more than 700MBps in sequential write operations. Performance is similar to the company’s F5402E Fibre Channel version: up to 750,000 I/Os per second. E5402E systems can be linked for support of up to 60 SAS/SATA drives.

Atto Technology’s ExpressSAS RAID adapters (models R380 and R348) support 3Gbps SAS and SATA drives and come in a low-profile form factor that enables users to fit them into standard PCI Express slots. The adapters support RAID 6 and RAID 60 (as well as most other RAID levels), both of which protect data in the case of dual drive failures, and are based on 800MHz PCIe IOP348 I/O processors from Intel. In addition to Windows and Linux, ExpressSAS adapters support Mac OS X platforms. MSRP is $1,095.

Overland Storage’s Ultamus 1200 and 5200 RAID arrays support SATA drives, with support for SAS drives expected later in this quarter. The Ultamus RAID 1200 array can be configured with up to 12 drives for 6TB of capacity using 500GB SATA drives. The model 5200 can be configured with up to 52 drives for a capacity of 26TB in a 4U form factor.

The Ultamus RAID arrays support RAID 6, and both systems come with four 4Gbps Fibre Channel host connections, as well as snapshots for point-in-time backup/restore. The models 1200 and 5200 are priced from $15,000 and$36,000, respectively.

Dot Hill’s 2U 2730 disk arrays have 4Gbps Fibre Channel host interfaces and can be configured with up to 56 SAS or SATA-II disk drives via SAS-based expansion units. Each 2U 2730 system includes 12 front-accessible drives (up to 6TB total capacity) and two active-active controllers. Each controller has two 4Gbps Fibre Channel host ports, and SAS or SATA drives can be used on the drive side. Resellers of the 2730 include vendors such as Hammer PLC, Maximum Throughput, ONStor, and Stratus Technologies. The array is based on Dot Hill’s R/Evolution storage architecture.

An AssuredSnap feature provides the ability to restore a data set to a specific point in time. User-configurable cache policies enable performance tuning and improved cache utilization. Rolling firmware upgrades allow upgrades installed on one controller to be automatically migrated to the second controller.

STORServer is shipping the K6000 backup appliance. The K6000 is based on Qualstar’s XLS LTO tape libraries, which have 300 to more than 6,000 tape cartridge slots. The appliances also include IBM x3800 or x3850 series servers and either internal SAS or external SATA storage shelves, all racked inside a Qualstar XLS. The K6000 is available with Tivoli Storage Manager Enterprise Edition and Progressive Backup Methodology software.

The K6000 provides a 10TB cache pool for storing backup data on disk for rapid recovery. Data is migrated to tape, and disaster-recovery copies are made automatically using the appliance’s management software.

AMCC is expected to enter the SAS market in May with the introduction of a SAS RAID controller based on the company’s SATA StorSwitch and PPC architectures. The controllers are expected to support RAID-6 parity generation and provide 700MBps to 800MBps of sustained performance. They will support up to 128 SAS or SATA drives per controller and up to four controllers per SAS domain. -InfoStor staff

For charts and graphics related to this article, go to SAS: The new kid on the I/O block.


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