A whirlwind of storage

New storage connectivity technology is on the horizon that promises faster performance, greater scalability and more flexibility for companies of all sizes.

The two technologies expected to take centre stage early next year are Serial Advanced Technology Attachment (SATA) and Serial Attached SCSI (SAS). SATA and SAS are point-to-point connections for attaching servers to internal or closely located storage. The SATA and SAS connections consist of a controller, which fits in a PCI, PCI-X or PCI Express slot in the PC or server and attaches to ATA or SCSI drives.

Fujitsu Ltd., Seagate Technology LLC and Maxtor Corp. are expected to introduce SATA drives this year; IBM Corp., Dell Computer Corp. and Hewlett-Packard Co. will introduce PCs and servers that integrate the SATA specification early next year. SAS products will start to appear in 2004, vendors say.

Analysts say they expect SATA and SAS controllers and drives to replace the current parallel ATA and parallel SCSI used now in servers and workstations. They say SATA will be used in entry-level to midrange servers where cost is an issue; whereas SAS will be implemented in high-end and midrange servers running business-critical data centre applications.

Gartner Inc. expects the number of ATA/SATA drives to exceed 9.2 million in 2006 from 2.4 million drives this year. International Data Corp. (IDC) in Framingham, Mass. estimates that SATA drives will represent nearly 100 per cent of all ATAs shipped in 2005.

Sated with SATA

The SATA connection is designed for reliable operation in servers that are hosting static Web pages or running file/print applications and to replace parallel ATA implementations. Parallel ATA, analysts say, has reached the end of its capabilities.

The technology was introduced in the 1980s and is the standard storage interface for PCs and servers. It is a relatively simple interface that performs as fast as 66MBps. But as dominant as parallel ATA is, it suffers from three limitations that become more important as servers become faster and more robust.

Parallel ATA uses five-volt signalling, which limits its use as companies try to reduce chip voltages and the use of large chip pads. It uses an 18-inch-long, two-inch-wide ribbon cable with 40 pins, which, when squeezed into a server chassis, severely restricts the available space for installing peripheral adapters and disk drives and restricts airflow, thus causing heat problems. Parallel ATA also has data protection issues that limit its reliability – it uses only a cyclic redundancy check and performs no checking of the ATA command data.

“Parallel ATA was never designed to be used in transaction-rich environments where you want all the error correction enabled that you can,” says Dave Reinsel, research manager with IDC.

Instead, Reinsel says, it was used in environments where one drive was attached to a server and because of the applications run on it, was not as robust from an error correction point-of-view as SCSI was.

With SATA, the cables that attach drives to the server are thinner, longer and more flexible – they are one metre long, much thinner than the customary ribbon cable and have a six-pin connector – making it easier to install. SATA also supports hot-pluggable drives, letting IT managers replace failed or troublesome drives without taking the server down.

IT managers say SATA will be a boon to server configuration and installation.

“If someone can replace the ribbon cables in machines with a thin, round, serial cable, I’m all ears,” says Jeff McMillan, the owner of McMillan Consulting in Olympia, Wash. “I fight with ribbon cables in PCs all the time; they are cumbersome, typically too short, accommodate too few devices and have connectors that I’m always afraid I’ll pull the ribbon out of the connector. They are difficult to seat, especially when light is dim. If you spread drives around in the box, it’s a real pain to make the connections reach.”

Another IT manager says SATA use will make a difference in configuring his servers, and also will boost server performance and troubleshooting.

“I’m interested in SATA for its speed and backward-compatibility with legacy ATA devices, also its hot-plug capability,” says Randall Dalton, systems engineer for Bibb and Associates in Lenexa, Kan. Bibb and Associates is a large architectural and engineering firm that is owned by Peter Kiewit Sons, a US$3-billion company. Bibb has 25 Windows NT 4, Windows 2000 and NetWare 6 servers.

SATA will be delivered into two phases: SATA I and II. SATA I improves the use of SATA devices in server and storage environments by supporting hot-pluggable drives and providing longer cabling, improved reliability and higher performance.

SATA II adds additional features and increases performance from 150MBps to 300MBps. SATA II devices, which will be backward-compatible to SATA I, will be available next year. SATA, with a maximum data transfer rate that will increase to 600MBps over time, also is backward-compatible with parallel ATA devices.

Getting sassy with SAS

SAS, on the other hand, will be used in servers running mission-critical applications and, in many cases, analysts say, as an alternative to deploying Fibre Channel storage area networks (SANs). It will replace parallel SCSI, which is at the end of its life in terms of performance and scalability, Reinsel says. Parallel SCSI performs at a maximum of 320MBps and only at half-duplex; SAS achieves bandwidth of as much as 600MBps and operates in full-duplex mode, in which data flows bidirectionally.

“Serial Attached SCSI is a different animal,” Reinsel says. “It is strictly an enterprise play, designed to replace parallel SCSI and to compete with Fibre Channel and SATA.”

SAS, because it is relatively inexpensive and easier to install than Fibre Channel, will replace SANs in some instances, Reinsel says. Customers who want to deploy small point-to-point Fibre Channel SANs, in which one array attaches to one server, will be attracted to SAS.

Furthermore, customers are familiar with SCSI and not Fibre Channel.

“Companies are going to be more likely to invest in legacy-proven technology such as Serial Attached SCSI and in products they have installed and where they have expertise, more so than some of the latest, greatest and newest technologies, because they have cost issues they need to solve,” Reinsel says.

IDC says SAS drives will account for about 50 per cent of the market by 2005.

SAS uses the same connector style as SATA, and the same signalling and encoding, making it a reliable choice for customers who don’t want to install Fibre Channel. It supports all the features of parallel SCSI and has better performance and reliability, while remaining backward-compatible with SATA devices. It also is scalable outside the box to 128 devices rather than the 16 devices that parallel SCSI supports.

“I’d likely [use SAS] in places where we do not want to spend the money on Fibre Channel, assuming SAS comes in cheaper than Fibre Channel,” says Geoffrey Carman, senior technical support analyst at York University in Toronto.


Is SATA ready for prime time?

With a top speed of 150MBps per drive, SATA exceeds ATA’s performance ceiling (133MBps), and its long-term roadmap tops out at 600MBps. On specifications, SATA is closing on SCSI and may eventually overtake it in some respects. But is SATA ready now?

A few system motherboards and PCI controller cards are on the market. One drive manufacturer, Seagate Technology LLC, is shipping a 120GB SATA drive with a street price of less than US$200. Controller maker 3Ware Inc. sells a 12-port SATA RAID controller with a 33MHz, 64-bit PCI interface for around US$800. Several other manufacturers plan to implement SATA; the list of SATA Working Group founders includes Maxtor Corp., Intel Corp., and Dell Computer Corp.

What keeps SATA from taking the market by storm is that it cannot replace either ATA or SCSI. SATA is strictly a hard drive interface; optical, tape and removable media devices aren’t supported. SATA is poorly suited to the backplane designs used for internal hot-pluggable drive arrays. If your system handles 12 drives, you’ll have 12 SATA connectors and 24 individual cables (one each for power and data) to sort out.

Drive manufacturers have little incentive to build enterprise-grade Serial ATA devices. More SATA versions of ATA drives are sure to appear this year, but even the best of these falls well short of SCSI standards. Most SCSI drives are rated for continuous use and carry five-year warranties. ATA drives are built for lighter duty, and their three-year warranties (some have been reduced to one year) reflect that.