Published: 04 Oct 2005
With a wave of new serial attached SCSI (SAS) products about to be released, some storage vendors are ratcheting up campaigns that promote SAS as the inevitable next step in the evolution of SCSI. Vendor interest is clear: to drive new sales and provide an alternative to Fibre Channel (FC) for organizations refreshing their disk storage. Some users are also jumping on the SAS bandwagon, although few have actually gotten their hands on SAS products.
SAS brings some FC-class capabilities and other advances to SCSI, including:
- Faster disk rotation speeds -- 10K RPM and 15K RPM
- Dual ports -- the ability to connect the drive to two communications links
- Serial cabling -- more flexible; easier to work with cabling to the disk
- Duplex operational mode -- bidirectional access to the disk
- Greater throughput -- 3 Gbps (6 Gbps duplex) and slated to double quickly
- 2.5-inch form-factor option -- allows more drives to be packed into a storage cabinet
- Low price, transparent transition -- prices expected to be comparable with enterprise SCSI drives
"We want the 10K RPM and 15K RPM disks that SAS will provide. As our databases grow, we want that extra performance," says Josh Wopperer, network engineer at UVEST Financial Services, Charlotte, NC, a provider of third-party brokerage and investment services to financial institutions in 42 states. Currently, the company stores its databases on an EqualLogic Inc. SAN populated with slower (7,200 RPM) serial ATA (SATA) disks.
With 500 people hitting UVEST's databases simultaneously, the SATA drives are capable of handling the traffic. Looking ahead, however, Wopperer can imagine a time when 2,000 users are accessing the databases and running complex queries. At that point, he anticipates performance problems and faster drives could make a noticeable difference. For storage managers, the switch to SAS from SCSI can be a transparent transition, which is the approach most are expected to take, or they can also devise strategies on how best to deploy some of the new features such as dual porting, increased drive density and high disk speeds.
"In the first phase, IT can treat SAS simply as a parallel SCSI replacement," says Harry Mason, president of the SCSI Trade Association (STA) Board of Directors and director of industry marketing at LSI Logic Corp., a Milpitas, CA-based maker of SAS components. In this case, SAS is a no-brainer because vendors are committed to maintaining SAS pricing that's comparable with existing SCSI and ensuring plug-and-play compatibility.
An interchangeable serial interface with SATA opens up even more possibilities. Because SAS controllers are compatible with SATA drives, "we will be able to mix SAS and SATA drives within the array," says Paula Long, co-founder and vice president of products and strategy at Nashua, NH-based EqualLogic. EqualLogic plans to incorporate SAS drives in its products by early 2006. Storage administrators like Wopperer could then implement a tiered storage strategy within the array or SAN, sending high-demand data and large databases requiring top performance to the faster, more costly SAS disks while steering the rest of the data to less-expensive, lower performing SATA disks. It's been a four-year journey for SAS as the storage industry has scrambled to find a replacement for parallel SCSI, which was hitting the wall in terms of throughput and device connectivity. By switching from a parallel interface to a serial interface, and then making it backward-compatible with the serial interface for ATA drives, storage vendors are able to shoot past the 15-device limit for the parallel SCSI bus and double the 320MB/sec top-end performance of parallel SCSI. They can also make a number of other good things happen.
The smaller, more flexible serial cabling lets manufacturers pack more drives into a cabinet and, at the same time, improve air flow and cooling capabilities. For even greater density, SAS drives will be available in two form factors: 2.5 inches as well as the typical enterprise 3.5-inch disk drive. "The 2.5-inch drive is one-third the overall size of a 3.5-inch disk, which means you can put more disks in the same physical space," says Mike Chenery, vice president of advanced product engineering at Fujitsu Computer Products of America Inc. The availability of 2.5-inch form-factor disk drives creates some interesting opportunities, although most corporate storage managers who opt for SAS as their SCSI replacement will go with the familiar 3.5-inch disks.
"Some of this is a defensive measure by vendors. They need things like dual porting to compete with Fibre Channel disk drives," says Arun Taneja, founder of the Taneja Group, Hopkinton, MA. Dual ports, for example, ensure high availability by providing an alternate connection to the drive.
First products roll out
The storage industry promised delivery of the first SAS storage products by the second quarter of 2005. And on June 22--with eight days left in the quarter--Fujitsu unveiled what it claimed as the first enterprise-level SAS array. Dubbed the Supermicro M28E, the product consists of eight 2.5-inch 36GB or 73GB Fujitsu drives and a host bus adapter (HBA) by LSI Logic inside a container provided by Supermicro Computer Inc. The product is priced at $4,620 with 500GB of capacity; the 1TB system is $8,400.
The Fujitsu product is just the start of what STA's Mason expects will become a flood of SAS storage product announcements in the coming months. "Previously, we've been hearing announcements of different pieces and parts," he says, referring to a steady stream of announcements of introductions of SAS controllers, HBAs and an entire gamut of storage components from companies like Adaptec Inc., Ario Data Networks Inc., LSI Logic and others. "Now we'll start to see the full system announcements from the storage vendors," he adds.
"I don't know of any server vendor who isn't looking to replace SCSI disks with SAS," says Taneja. Among the storage vendors, all those with SCSI systems will invariably turn to SAS. "Only vendors like EMC, which have been into Fibre Channel for a long time, may not do much with SAS, at least not initially," adds Taneja.
According to the performance specs accompanying the Fujitsu announcement, the new arrays can handle 3Gb/sec. Parallel SCSI drives currently top out at approximately 320MB/sec due to the parallel nature of the architecture. From a technical standpoint, SAS' link rate of 3Gb/sec delivers 300MB/sec--this is according to "What's faster, Serial Attached SCSI or Ultra320 SCSI?," a paper for the SCSI Trade Association written by Bob Sheffield, a senior staff storage architect at Intel Corp.
However, the differences between parallel and serial architectures, as well as the full-duplex nature and dual porting of SAS drives, make straight speed and throughput comparisons between SCSI and SAS difficult and not useful. For example, because of duplexing, a 3Gb/sec SAS drive can actually handle 6Gb/sec, Mason explains. With four ports running in full duplex mode, the throughput could be pushed to 24Gb/sec, far exceeding what could be achieved with a comparable number of parallel SCSI drives.
Furthermore, the performance of SAS is just beginning at 3Gb/sec. The SAS roadmap will take the technology to 6Gb/sec by 2007 and to 12Gb/sec a few years after that. "The best way to think about performance is that the SAS serial architecture, coupled with silicon and drive advances, produces significant benchmark gains over the best parallel SCSI," says Mason. By comparison, 4Gb/sec FC drives are just being introduced this year, but exactly where the industry goes from there and when it will happen isn't clear.
A closer look at SAS
SAS takes full advantage of the serial architecture. It can address up to 4,032 devices per port through a point-to-point approach that operates initially at 3Gb/sec, but will quickly increase from there. The point-to-point architecture is the preferred approach to implementing high-availability systems, STA's Mason explains. SAS also capitalizes on advances in chip technology, which have produced integrated circuits that handle multiple, point-to-point drive connections within a single device.
To take advantage of these drive connections, SAS uses expanders, which are built-in switches. The expanders enable the necessary fan-out to create large, enterprise-class configurations while maintaining backward-compatibility with today's existing, proven SCSI software, Mason adds.
Low-cost SATA drives currently spin at 7,200 RPM. The new SAS drives offer 10,000 RPM and 15,000 RPM options. This puts them squarely into the enterprise FC class, able to handle high-volume, complex database transactions, as UVEST's Wopperer notes.
Dual porting, another enterprise feature, brings increased high availability to the storage system. Each drive can have an alternate link, maintaining connectivity should one link fail.
Full duplex refers to the ability for simultaneous transfers outbound and inbound, unlike parallel SCSI, which allows only unidirectional transfer, according to Intel's Sheffield. This ability, he writes, "is particularly useful in targets with large data caches running workloads that yield high cache hit rates, yielding aggregate bandwidths approaching the theoretical 600MB/sec."
Physical serial cabling, as mundane as it seems, provides a big advantage over parallel SCSI. "There were a lot of problems with parallel cabling," says Taneja at the Taneja Group. Beyond a theoretical limit of 15 devices, the wide, unwieldy parallel ribbon cable made it difficult to pack multiple drives in a cabinet. With the flexible, round serial cable, vendors can squeeze in more drives while still ensuring sufficient air flow for cooling. In some cases, they'll even be able to use smaller fans, further lowering the cost of the storage system.
By combining smaller SAS drives with lower capacity (36GB and 73GB), vendors will be able to pack cabinets with more drive spindles. "There will be more arms, more spindles, which will give more performance," adds Fujitsu's Chenery. Over time, vendors expect SAS disk capacity to increase; however, today's large SATA drives present their own problems, such as slow RAID rebuilds in the event of a failure. Finally, the ability to connect SATA to SAS drives using the same connectors will enable companies to deploy what amounts to tiered storage in a box. SATA drives are able to plug directly into the SAS connector. However, a small piece on the connector prevents a SAS drive from plugging into a SATA connector. This isn't too important to large enterprises because they are already mixing SCSI and SATA with separate boxes, says Taneja. Small- and midsized companies aren't likely to buy multiple boxes with different classes of disks, he adds, so mixing drives with different performance and price characteristics in the same box would be more appealing.
The future of SAS
As SAS spreads throughout the industry over the next year, it will be used mostly as a replacement for parallel SCSI. Over time, Mason expects SAS to enable some interesting things in the areas of clusters, blade servers and high-performance computing. For example, SAS might be used to build clusters, blade servers and blade storage servers. In such a case, SAS could be used to share boot drives across multiple blades. SAS, in effect, would act as a storage midplane. "It would be a way to preserve the SCSI boot model, rather than having to boot from the SAN," Mason explains. A variation of this could be used for high-performance computing, he adds. SAS products have finally arrived, and transitioning from SCSI to SAS should be seamless. Organizations will bring in SAS drives as they replace existing SCSI drives, which the industry will slowly phase out over the next decade. The real changes lie ahead, when the industry starts to take advantage of the specific capabilities of SAS -- in particular, what appears destined to become a near universal serial storage interface to provide more ways to network storage.
About the author: Alan Radding is a frequent contributor to Storage magazine.