One of the key advantages of Serial ATA (SATA) and Serial Attached SCSI (SAS) is that they were designed to work together.
By Mark Ferelli
Set the WayBack machine to 1979. Shugart Associates, led by storage industry pioneer Alan Shugart (who later founded what became Seagate Technology), created the Shugart Associates Systems Interface (SASI). This early ancestor of SCSI was rudimentary by comparison with subsequent interfaces. For example, it supported only a limited set of commands and a slow signaling speed of 1.5MBps. However, for its time SASI was a truly disruptive technology, since it was the first step toward an intelligent storage interface for small computers. The limitations of the interface must be considered in light of the technology level at the time: 8-inch floppies were common, and networked storage was unheard of.
In 1981, Shugart Associates teamed up with NCR and convinced the American National Standards Institute (ANSI) to set up a committee to standardize the interface. A number of changes were made to the interface to broaden the command set and improve performance.
The name was also changed to the Small Computer Systems Interface (SCSI), which made the interface less proprietary, according to many people. The actual standard was published in 1986, and evolutionary changes to the interface have been occurring ever since.
It’s important to remember that SCSI is, at its heart, a system interface, as the name suggests. First used for hard disk drives, SCSI is still used predominantly for disk disks and is often compared with the IDE or ATA interfaces. For those reasons, SCSI is sometimes thought of as a hard disk interface. However, SCSI was never tied specifically to hard disks.
The interfaces currently enjoying the greatest mind share and market growth are Serial ATA (SATA) and the newer Serial Attached SCSI (SAS). One drive vendor recently reported that its last quarter’s shipments included 2 million SCSI drives, 105,000 SAS drives, and a whopping 9.6 million SATA drives.
SATA then and now
Initially, SATA was positioned as the evolutionary successor to low-end parallel ATA (PATA) devices, and significant enhancements were added in SATA 3G (SATA II) devices. These improvements have made SATA a contender for “enterprise” applications.
In a relatively short period of time, SATA has made significant inroads, not only in desktop applications, but also in server, NAS, SAN, and many diverse applications where cost/capacity and performance intersect.
SATA drives are expected to make up 23% of the overall enterprise disk drive market by year-end and are expected to account for about 34% of the market in 2008, according to International Data Corp. (see figure).
Very high performance is a key advantage of serial technologies such as SAS and Fibre Channel. But “good-enough” performance is available in the current generation of SATA devices.
“Historically, only the highest performance and capacity points were based on serial technology [such as Fibre Channel],” says Steve VonderHaar, vice president of sales and marketing at Ario Data Networks. “Today’s SAS and SATA drives remove that distinction by bringing serial technology across all tiers of storage. In addition to the serialization, the capacity points of SATA drives are changing the market drastically. In the early days of ATA versus SCSI, SCSI drives were often selected simply due to the capacity points. The playing field has changed as SATA capacity points are exceeding those of Fibre Channel and SAS drives.”
It’s not just a matter of capacity anymore. SATA is being used in a wide variety of high-performance applications. For example, Barbara Murphy, vice president and general manager at AMCC, notes that the company’s SATA controllers are being integrated in a broad array of applications, including video editing, digital content creation (DCC), broadcast, scientific, and industrial applications ”where high levels of sequential bandwidth are required at price points where SCSI, SAS, and Fibre Channel cannot compete.”
SATA’s performance road map started at 1.5GHz in the original specification, and 3GHz (3Gbps) is now commonplace, as are performance enhancements such as native command queuing (NCQ). “SATA was faster than expected, especially for those who were using Fast ATA,” says Tom Treadway, CTO at Adaptec, “and SATA is now available on 10,000rpm drives.” The next generation of SATA specifies a 6Gbps transfer rate (see figure).
It would be tempting to identify SATA as a SCSI replacement, and indeed many data centers are looking forward to adding less-costly arrays to their infrastructure as their capacity demands grow. However, Harry Mason, director of industry marketing at LSI Logic, believes that SATA is growing more into nearline applications and “is not eating into SCSI volumes.” (Mason is also the chair of the SCSI Trade Association.)
Mason also is concerned about end-user expectations. “Some customers misuse SATA,” he says. “They bring it into some applications because it’s cheaper, but if it’s used for applications like OLTP [online transaction processing], users can experience higher failure rates and slower speeds.”
This may well change, however, with the emerging category of so-called “enterprise SATA.” The new category is characterized by improved mean time between failure (MTBF) and reduced rotational vibration, which Mason says have been long-term historical problems with SATA.
SAS follows a different drummer
About five years ago, it became fairly clear that SCSI was reaching its performance limits. As a result, vendors of chips, drives, and subsystems developed what became the SAS interface. SAS uses a serial, point-to-point topology to overcome the performance barriers associated with storage systems based on parallel bus or arbitrated loop architectures. (In a point-to-point configuration, each device gets full bandwidth.)
SAS subsystems can also accommodate both low-cost, high-capacity SATA disk drives as well as high-availability, high-performance SAS drives. This ability to support two classes of drives offers users, systems/storage integrators, and OEMs new levels of flexibility in designing, scaling, and managing disk arrays.
In fact, the key advantage of SAS is choice. Moving to SAS lets users meet enterprise-level performance, capacity, and reliability requirements, while being able to leverage legacy SCSI software and mix-and-match SAS and/or SATA drives.
Pricing may be an issue with SAS, especially considering that SATA and SAS share the same mechanical characteristics with about the same cost of production. But the nature of the I/O is a differentiator. “SAS and SATA II devices will co-exist in storage hierarchies; however, we do not believe they will be deployed to a great extent within the same box because the nature of the I/O to SAS and SATA drives is so divergent,” says AMCC’s Murphy. “SAS drives are highly optimized for random, small-block transactions, and SATA drives are optimized for sequential, large-block I/O.”
“The [relative] growth potential for SATA and SAS will depend very much on how the drive vendors price the drives,” says Ario Data’s VonderHaar. “SATA is taking off today due to appealing current and projected price-per-gigabyte points, [but] different vendors have different motives, depending on market shares, and watching the ATA/SATA-focused vendors jockey for position with the vendors that have been historically strong in Fibre Channel and SCSI will be interesting.”
SAS, not SATA, seems to be the key SCSI replacement technology. “The initial implementations of SAS are focused on replacement technology for the traditional SCSI server market,” says AMCC’s Murphy. “SAS drives can be used in the same environments in which SCSI and Fibre Channel are used today-in enterprise environments where higher levels of random performance and redundancy are required. SAS as an infrastructure play will enhance SATA II deployment by enabling support for very high-capacity implementations.”
Although SAS is expected to account for only 5% of total enterprise-level disk drive shipments this year, it is expected to garner about 25% of the market by the end of 2008, according to IDC. In that same time frame, IDC expects SATA to represent 34% of the market, with 27% going to Fibre Channel and 14% to parallel SCSI disk drives (see figure on p. 22).
In addition to the advantages already mentioned, other features of SAS include dual porting, 16,000 devices per domain, “wide” ports, and rate matching.
The vision for SAS and SATA has always been for the interfaces to work together. In late 2003, Maxtor and LSI Logic were the first manufacturers to prove that data can be exchanged between SATA and SAS drives with the aid of SAS controllers and expanders.
“Generally, SAS and SATA are complementary from a systems perspective, but they serve different market segments and have unique benefits,” says Joel Warford, vice president of marketing and business development at SiliconStor.
“In enterprise storage array and server applications, SATA drives can be used in either SAS or adapted Fibre Channel system infrastructures as an alternative to SAS or Fibre Channel drives for the best cost-capacity in applications such as disk-based archiving, nearline applications, and many primary storage applications.”
An aggressive growth path for both SATA and SAS is expected, since the need for storage capacity is growing dramatically across the board independent of the drive interface choice. SATA is growing faster as a percentage of overall storage capacity due to the attractive cost-capacity point. SATA is positioned to dominate low-end markets (home/SOHO/small to medium-sized business), some medium-range markets (NAS/SAN), and in “nearline” backup/recovery/archiving applications. And as noted earlier, the concept of “enterprise SATA” will challenge the dominance of Fibre Channel and higher-end SCSI drives as this market becomes more cost-sensitive and SATA becomes “good enough” for these applications.
Although the vision was to have SAS and SATA complement one another, SAS proponents may not have foreseen the impact of SATA II, a higher-performance implementation of SATA that provides features that put it in direct competition with SAS in many areas. For example, SATA II supports hot-swapping. This is significant in the development of enterprise SANs, as well as for smaller RAID subsystems and NAS devices. Because less-expensive SATA II drives can be swapped in and out of arrays, users don’t have to go with Fibre Channel, SCSI or SAS to hot-swap drives.
Additionally, SATA II specifies a port multiplier feature that enables connections to as many as 15 SATA or SATA II drives (although four or five drives connected to a single port multiplier provides optimal performance). A port multiplier can be connected internally to a server and deliver as much as 6TB of capacity, depending on the number of drives on a single channel. SATA II also includes an external cabling spec that can connect a server and SAN up to two meters apart. And a single cable can carry four channels of data.
“For more and more storage applications, IT budget constraints are driving the need for lower cost storage where ‘good-enough’ SATA drives compete with ‘better’ SAS drives,” says SiliconStor’s Warford. “SATA may prevail in many purchasing decisions. IT managers have to justify the additional costs for SCSI, SAS, or Fibre Channel drives.” SAS was developed to promote choice in the storage interface environment. The choices are there, but the selection process is as challenging as ever. Cost, performance, and reliability are still the key criteria, and the victor will depend largely on which of these criteria comes first.
Mark Ferelli is a freelance writer. He can be contacted at firstname.lastname@example.org.
For more information on SAS and SATA, go to the following Websites:
Serial ATA-IO Organization:
T13 (ATA spec development):
T10 (SAS spec development): www.t10.org
SCSI Trade Association (STA): www.scsita.org
Serial Storage Wire: www.serialstoragewire.com
Also check out the following articles in InfoStor, either in print or online at www.infostor.com
“SAS rollouts pick up steam,” June 2006, p. 8
“Five interfaces, five vendors, five opinions,” January 2006, p. 22
“SATA, SAS drive toward the enterprise,” November 2005, p. 22
“Do SAS-based SANs make sense?” October 2005, p. 8
Recent SAS-related product announcements
Here’s a sampling of SAS-related products that recently began shipping, with a focus on controllers and adapters. (Note: Some of these products were previously covered in the June issue.)
Adaptec recently expanded its SAS product family with two entry-level additions to its HostRAID controller line. The SAS 44300 and 58300 feature software-based RAID data protection, mini SAS connectors, and a low-profile form factor. The software supports RAID levels 0, 1, and 10 (mirroring and striping).
The controller cards support the 64-bit, 133MHz PCI-X host interface. Common features of both models include support for both SAS and SATA disk drives and support for up to eight direct-attached drives, or as many as 128 devices using SAS expanders. (SAS expanders allow for configurations in which multiple initiators can have connections to multiple targets.)
The lower-end model 44300 controller (priced at $250) includes one x4 (four-lane) internal connector, while the 58300 (priced at $360) has two x4 external connectors and is designed to attach to SAS JBOD or RAID arrays. Adaptec also sells 4-port and 8-port HostRAID SAS controllers that support the PCI-X and PCI-Express (PCIe) host interfaces.
Arena Maxtronic last month began shipping a variety of SAS subsystems. The SA-4342S is a 2U, eight-bay, SAS-SATA II RAID array with an Intel i80331 CPU. The SS-4551S (4Gbps Fibre Channel-SAS) and 4552 (SAS-SAS) are 2U, 12-bay RAID arrays that include an AMCC PPC440SP processor. And the SS-6651S, 6652S, and 6652J are 3U, 16-bay arrays based on AMCC’s PPC440SP CPU in 4Gbps FC-SAS or SAS-SAS RAID or JBOD configurations.
Ario Data Networks recently began shipping the first two products in its Capacity Storage Array (CSA) series of SAS and SATA disk subsystems. The Capacity RAID Array (CRA) and Capacity Expansion Array (CEA) feature a front-loaded blade design that allows disk maintenance without interrupting operations. The CSA family is based on Ario’s RAID controllers and JBOD SATA and SAS connectivity products.
Each 4U CSA array can hold up to 13 StorBlades that contain four SATA II or SAS drives each, for a total capacity of up to 26TB. The StorBlades are accessed from the front of the array, and in case of a failed drive, Ario’s SafeBlade technology stores I/Os while the blade is removed. When the blade is replaced, the system rebuilds itself in the background.
The CSA arrays have a modular design that allows expansion by swapping out the RAID controller for a JBOD controller. Users can also add I/O connectivity modules for Fibre Channel and 1Gbps or 10Gbps iSCSI.
Atto Technology has developed a line of SAS/SATA RAID adapters for PCIe hosts. The low-profile ExpressSAS adapters provide up to 2.4GBps of throughput and include user-selectable port configuration options. The adapters support RAID 1/0/5/6/10/50/60 and include a DVRAID feature for applications such as 2K and 4K film, real-time effects, and high-definition video editing. The ExpressSAS RAID adapters are based on x8 PCIe technology. Other features of Atto’s SAS adapters include hot-swap spares, online capacity expansion, and battery backup.
Infortrend’s EonStor S12F-R/G1420 arrays have one or two RAID controllers with two 4Gbps Fibre Channel host connections per controller, up to 4GB of cache, and as many as 12 3.5-inch SAS or SATA disk drives in a 2U enclosure. In a four-channel configuration with RAID 5, the company claims sequential read performance of 780MBps and sequential write performance of about 521MBps. All components are redundant and hot-swappable.
Infortrend also sells the EonStor S12S-J1000, a SAS-to-SAS JBOD subsystem with redundant SAS expander boards.
LSI Logic last month began shipping low-profile SAS RAID adapters. The MegaRAID SAS 8308ELP and 8344ELP PCI Express adapters have eight SAS ports and support for 3Gbps SAS and/or SATA II disk drives. The adapters are optimized for direct-attached storage (DAS) server configurations.
The MegaRAID SAS 8308ELP has eight internal ports, while the 8344ELP has four internal and four external SAS connectors. Both include remote battery backup options to protect data stored in memory. A new feature in the software for the adapters-Mega-RAID Patrol Read-scans attached disk media for bad blocks and prohibits further writing of data to those blocks to improve reliability and data integrity. LSI claims to have shipped more than 500,000 SAS components to the server OEM market.
Promise Technology has announced a variety of SAS-based disk arrays and RAID controllers, including a PCI-Express-to-SAS RAID controller that can connect up to 128 disk drives via expanders; a 4Gbps Fibre Channel-to-SAS VTrak RAID array; and the VTrak J3002 SAS JBOD storage system.
Features of Promise’s 2U VTrak J3002 include 12 hot-swappable drive bays, optional dual active/active SAS JBOD I/O modules, three 3Gbps SAS 4x external ports (two host ports and one SAS expansion port for cascading subsystems), and fully redundant components. The subsystem supports 12 3.5-inch, 3Gbps SAS or SATA drives in a 2U rack. Up to four systems can be cascaded for more than 24TB of total capacity.
Rasilient Systems last month began shipping the Rastor 6000 RAID array, which supports SAS and SATA drives. The 3U array can be configured with up to three 2U, 12-bay SAS expansion JBOD arrays for capacity extension up to 51 disk drives.
Rasilient’s subsystems, which are designed primarily for OEMs, can be configured for IP SAN, Fibre Channel SAN, SAS, or NAS. Other features include active-active controller fail-over and redundant, hot-swappable components.
Designed for embedded applications and OEMs in the telecommunications market, SBE’s 6U SAS iBlade supports iSCSI and 8 or 16 3Gbps SAS or SATA ports on a CompactPCI-compatible board. SBE ported the PyX iSCSI software stack (with Error Recovery Level 2, or ERL2) to its HW400c/2 core-processing platform and bundled it with Astek’s PMC-based SAS host adapters.
Xtore, a business unit of Advanced Industrial Computer (AIC), has launched the XJ-SA26-224R-S, a 2U high-density JBOD array that supports up to 24 2.5-inch small-form-factor (SSF) SAS or SATA disk drives for a total capacity of up to 3.5TB. The company claims a bandwidth of 4.8GBps. The array includes four host connections and two cascade connections for attachment to capacity expansion units. A dual expander is optional.
AIC is shipping a number of storage enclosures that support SAS. Most recently, the company introduced the BR-SAS28 canister for 2.5-inch SAS/SATA drives and the RMC3E-SA chassis for 3.5-inch drives for up to 1.1TB of capacity. The company claims a bandwidth of 1.2GBps.
AIC’s RMC3E SAS server chassis is a 3U, 16-bay enclosure that supports 3.5-inch SAS or SATA drives and a maximum capacity of up to 8TB with SATA drives. The company claims a bandwidth of 4.8GBps.
On the semiconductor front, Vitesse last month began sampling SAS expanders in its NexSAS line. The second-generation VSC7156/57/58 expanders include features such standard phy and address-resolved zoning, a multi-affiliation STP/SATA bridge, integrated Ethernet MACs, dynamic multi-level equalization, programmable edge rate control, and a 32-bit RISC CPU.
Vitesse is also shipping SAS/SATA RAID-On-Chip (ROC) controllers-the VSC7250 and VSC7251-for the PCI Express host bus. The controllers can be configured as RAID controllers or SAS I/O controllers and include four or eight drive connections, an embedded RAID processor, RAID-5 hardware acceleration, and support for virtually all other RAID levels.
Molex is shipping a variety of SAS/SATA devices, including interconnects, guide frames, and cable assemblies that support 3Gbps and next-generation 6Gbps SAS, as well as 1.5Gbps SATA and 3Gbps SATA II. The products include the iPass interconnect system, LaneLink and x4 Internal Multi-Lane connectors and cable assemblies, and x1 SATA connectors.
Finally, all of the leading enterprise-level disk drive manufacturers are shipping SAS drives, including Fujitsu, Hitachi Global Storage Technologies, and Seagate Technology (which recently completed its acquisition of Maxtor).