SATA disk drives; 3 versus 1.5 gigabit/sec: dispelling the myth of higher interface rates.

Next-generation Serial ATA products promise to double hard drive interface performance from 1.5 gigabits per second (Gb/s) to 3Gb/s and possibly beyond, providing a 10-year performance roadmap for OEMs and channel system builders. But is 3Gb/s technology necessary across both the host and drive interface to deliver maximum data throughput for today's storage environments?

For host interfaces, the 3Gb/s speed is necessary to meet the increasing performance requirements for bandwidth-intensive applications such as IT and corporate data centers, health care service, manufacturing and financial real time systems, and nearline data storage as the amount of data that companies need to store, manage and keep readily available continues to increase. Serial ATA can help meet rising data throughput needs by doubling the speed of the data delivery path, enabling the transfer of more than 1.5 gigabits of data in aggregated arrays and other multi-drive configurations. From the host side, Serial ATA 3Gb/s essentially provides a larger pipe to move more data.

Benchmark tests today show that, in multi-drive configurations, 1.5Gb/s SATA disk drives can reach data transfer speeds of >200 MB/s when their individual maximum data transfer rates are aggregated into a single pipe to the PC or server host. Keep in mind the individual drives may be comparatively lower but the array performance is limited only by the number of drives in the array and by the speed of the underlying host interface rate. For the best performance, large numbers of drives can be connected and their data aggregated into a larger host interface.

[FIGURE 1 OMITTED]

Multiple drives connect to a 1.5 or 3Gb/s Serial ATA Port Multiplier. The collective drive data rate with the 3GB/s Serial ATA Port Multiplier equals 240MB/s into the host controller. By contrast, the collective drive data rate in a single-drive environment would be ~60MB/s.

Individual disk drives, however, rarely take advantage of 3Gb/s bandwidth. Why? An ATA drive's internal data transfer speed tops out at about 60MB/s--the maximum rate that the data head can sustain reads or writes to the disk media. The disk data rate depends on the number of bits packed onto a square inch, usually called areal density. Areal density is a combination of the number of bits on a track and the number of tracks within a square inch. The greater the areal density, and more specifically bits per track, the higher the data rate performance. To achieve higher data rates, drive manufacturers must add more bits into a square inch on the media.

The drive internal data rate results from the number of bits that can be transferred between the drive media and the drive head in a given period of time. The storage industry has done a fantastic job of ensuring the disk interface rate (formerly Parallel ATA, now Serial ATA) does not bottleneck the disk data rate. No matter how fast the interface, the drive can transfer only as much data as the drive can retrieve from the media--its sustained data rate. This means a single drive with a 1.5Gb/s interface connected to a 3Gb/s host interface delivers the same internal drive data rate as an otherwise identical drive with a 3 Gb/s interface, since the drive's internal data rate, not the interface, is the gating factor in performance.

SATA 3Gb/s (300MB/s) host interface provides bandwidth for RAID 0, 4-drive performance and more. The test results below are achieved by using a Silicon Image 3Gb/s Port Multiplier and 2--4 Seagate Barracuda 7200.8 SATA 1.5Gb/s, 400GB disk drives. Just 4 SATA 1.5Gb/s drives saturate a 1.5Gb/s interface and start to encroach upon the 3Gb/s interface limitations.

A 3Gb/s interface on the disk drive can, however, increase system performance when the application takes advantage of the drive's cache burst ability. In the case of writes, if the drive's write cache capability is enabled, any write that fits in the buffer will be transferred at the full 3Gb/s into the buffer. In the case of reads, caching helps when the data requested is small enough to fit into the drive cache and is present in the cache at the time of the request. That data will be transferred to the host at the full 3Gb/s rate. In either case, the data must be in the form of small sequential or near sequential files such as those found in some video editing applications. The reason: Small sequential or near sequential file transfers require less mechanical movement or seek overhead and maximize caching capabilities of the drive. In these types of applications, 3Gb/s drive interface rates can greatly increase performance.

While the 3Gb/s interface rate on a single disc is not critical to maximizing the performance of today's standard desktop computing systems, continuing increases in drive capacity and higher internal data rates driven by increasing disk areal density will create a need for greater interface bandwidth on both the drive and host. Technology that increases areal density will benefit consumers by increasing the amount of data on a single disk platter. More data on less platters means great reliability, less parts, and continued low cost per GB. Consumers get a whole lot of technology and capacity at a very low price.

One way drive makers plan to accelerate areal density and data transfer rates is to use a new hard drive technology called perpendicular recording. It increases the amount of data that can be packed onto a square inch of the drive platter, known as the drive's areal density (bits per inch X tracks per inch), by standing the bits vertically rather than lying them down horizontally. Bits make up sectors and sectors hold your data. Sectors are written on tracks very similar to a record player in circular layers around the media disk. With Perpendicular recording, the bits standing vertical instead of horizontal will allow more room for more sectors or more of your data. Perpendicular recording promises to increase storage on a single disc drive platter from gigabytes to eventually terabytes, which in turn will require much larger pipes to prevent a data bandwidth bottleneck and spawn a need for faster interface rates in single-drive environments. Several drive makers are expecting to offer perpendicular recording hard drives within the next year.

Serial ATA 3Gb/s will clearly increase performance when system builders take advantage of aggregated bandwidth or multi-drive configurations. But in single-drive environments, the advantages are more limited until the drive data rates increase. Why, then, are motherboard, host controller and drive makers adopting the technology across the board today?

The storage industry historically has avoided performance bottlenecks by ensuring the interface speed exceeds the data rate of the disk drive. The interface rate (1.5Gb/s or 3Gb/s) must never be allowed to constrain the total disk data rate. With more desktop systems taking advantage of aggregated bandwidth between at least two drives-RAID on board, there is currently a need to expand the host interface rate. In addition, video editing, a prime application for 3Gb/s hard drives, is no longer an experts-only application. Many systems ship with some sort of digital editing application as a standard feature; while not as widespread as email or surfing the web, digital video editing is a rising trend. This will drive the adoption of faster drive interface rates as well.

When considering the SATA interface options in systems today, it helps to keep this simple plumbing picture in mind: Disk drives have relatively small internal pipes, able to transfer ~60MB/s to and from the disk. The interface pipe between the disk and the controller, or host, is capable of transferring 1.5Gb/s today, and 3Gb/s in upcoming SATA systems. If there is only one drive in the system, it doesn't matter how big the interface pipes are; the system will only be able to pump through data at the rate carried by the smallest pipe (caching situations excepted). If there are multiple drives in the system, the larger interface pipe can carry more data provided by the multiple drives simultaneously, and as the numbers of drives feeding that pipe multiply, more advantage will be seen until the pipe is full. In that case, the advantages of the largest possible pipe will be most obvious, and host-side 3Gb/s SATA will provide system-level performance advantages over today's 1.5Gb/s SATA, regardless of the interface speed on the drive side.

Aggregated Multi-drive Performance Advantages

RAID 0: 4 Disk Drives    215.16
RAID 0: 2 Disk Drives     88.28

Figure 2

Note: Table made from bar graph.

Joni Clark is product marketing manager and Marc Noblitt is senior product marketing manager at Seagate Technology (Longmont, CO)

www.seagate.com