eSATA drive interface scales beyond PCs

External SATA (eSATA) provides a number of advantages over PC drive interfaces such as USB and 1394a.

By Conrad Maxwell

Digital content is pervasive today, from MP3 files to digital photographs and video, not to mention the mountains of e-mail and business records stored on both home PCs and corporate servers. The explosion of all of this digital information is rapidly consuming hard disk drive (HDD) space and creating a critical storage challenge.

The proliferation of digital data is the driving force behind external storage. Many home and small business users prefer the convenience of attaching an external HDD to address their expansion needs. These drives are typically removable and portable, allowing data migration or remote storage of backups. Evidence of this demand can be seen through the sales of external hard drives for PCs, which double every year and outpace sales of internal drives.

Today, these external drives are typically connected to a PC with a USB or 1394a (FireWire) interface. However, Serial ATA (SATA) will soon change this. According to John Monroe, research vice president at the Gartner consulting firm, “By the end of 2007, SATA should achieve 100% penetration in all desktop and notebook HDD markets.”

Recently, the SATA International Organization (SATA-IO) ratified the specification for external SATA (eSATA), paving the way for a new class of external drives. There are compelling reasons for the PC market to move to SATA for external applications, most notably the price and performance advantages over the USB and 1394 interfaces. External SATA can also be used for networked storage applications, which for the first time enables desktops and servers to use a common external storage interface.

The table provides a comparison of the raw interface speeds and data-transfer rates over the three PC interfaces-USB 2.0, 1394a, and SATA. SATA has a raw interface speed of 1.5Gbps-more than 3x that of USB 2.0 and 1394a-and 3Gbps SATA drives double that performance advantage. With a sustained transfer speed of 40MBps to 50MBps on today’s SATA drives, USB 2.0 and 1394a throttle drive performance, add protocol conversion overhead, and consume CPU cycles, limiting performance. Burst transfer rates (when data is read from drive buffer memory) highlight the performance differences and are becoming more important with the larger caches on today’s drives. SATA burst transfer rates are nearly 3x higher than those of USB 2.0 or 1394a, which is expected, given the raw interface speeds.

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SATA drives will cost less than external USB or 1394 drives since they don’t require protocol translation chips and will be essentially identical to the high-volume internal SATA drives used in desktops today.

In the data center, external storage-either in a JBOD or RAID disk arrays-typically uses SCSI or Fibre Channel drives. USB or 1394 are simply too slow for these applications. While ATA drives are often used behind a Fibre Channel- or SCSI-connected RAID controller, no external ATA connection has been possible until now. SATA, with its hot-plug support and higher data rates, as well as new features such as port multipliers, is well-suited for external server or networked storage.

With the introduction of eSATA interconnects, external storage arrays can now be connected directly to a host SATA port, without requiring an expensive RAID controller that does protocol conversion. This will open the door to lower-cost, yet high-performance, SATA direct-attach storage solutions.

One common application for external storage is a dedicated SQL or Exchange server, where the standard practice is to use a mirrored data set for log files and a separate RAID-5 volume for database files-thus often prompting the need for an external direct-connected SCSI array. In most installations, the performance of a SATA disk array is sufficient and can be installed at a much lower cost.

Another common external storage configuration is a video-editing station, where a large number of disks are needed to stripe large data files with acceptable throughput. These are often SCSI arrays with a shared SCSI bus to each of the drives. A lower-cost solution can be achieved with a SATA host controller and eSATA drives. Since the drive connection is dedicated for SATA, the performance is often higher.

With the transition from parallel ATA to Serial ATA accelerating, the opportunity for employing native SATA hard drives as external solutions is clear. SATA provides both higher performance and lower cost than USB or 1394. External SATA solutions can be enabled with PCI cards or PCMCIA cards for notebooks today. Future systems will use motherboard-based eSATA implementations and ExpressCard form factors for even higher performance.

SATA-IO’s eSATA specification paves the way for the proliferation of high-performance, cost-effective, portable external storage devices. eSATA enables the use of a common external interface technology across both PC and server platforms, which will lower costs. This creates a number of new convergence opportunities SATA is uniquely equipped to meet-not only for PCs, but for networked storage as well.

Conrad Maxwell is a member of the SATA International Organization (SATA-IO) marketing workgroup and is a product marketing manager at Silicon Image.

This article was originally published on June 01, 2006