Second-generation Serial ATA hardware has been with us since 2005. That's a long, long time ago in PC terms. Back in 2005, motherboards supporting the then-fresh Serial ATA spec used Nvidia's nForce4 and Intel's 955X Express core-logic chipsets. AMD and Intel were just dipping into 64-bit waters, and then only with a single core at a time. Graphics chips had discrete pixel and vertex shaders that would be considered horribly inflexible by today's standards. Ultraportable Windows PCs started at a couple thousand dollars, not a few hundred. And you were blissfully unaware that John and Kate Gosselin even existed.
Yeah, so-called SATA II has been around for a while. A draft specification for its third-generation replacement was presented back in 2008, with the final spec published in May of this year. So what's new? Not a whole lot, to be honest. Native Command Queuing has gained a streaming command that can facilitate isochronous transfers. The SATA-IO governing body also promises better NCQ performance thanks to a new feature that enables "host processing and management of outstanding NCQ commands." Oh, and the host interface speed has been doubled to 6Gbpsthat works out to about 600MB/s with protocol overhead taken into account. The higher data rate is gen-three SATA's defining feature; the standard is referred to as SATA 6Gbps.
Getting excited about a faster hard drive interface isn't hard. However, our first taste comes in the somewhat unlikely form of a mechanical hard drive: Seagate's Barracuda XT.
"But the Barracuda XT is Seagate's latest flagship desktop drive," you say. The XT's 500GB platters spin at 7,200 RPM, and it has a healthy 64MB of speedy DRAM cache. All of that sounds very impressive, except for a few problems.
The first issue is made quite obvious in the picture above. Yes, that's Seagate's top-of-the-line Serial ATA hard drive. The XT couldn't possibly look more generic, and there's really nothing to differentiate it visually from a bottom-rung Barracuda. Style may not count for much in a product that'll be buried inside a case, out of view of even most side-panel windows, but it should count for something on a $300 hard drive.
|Maximum external transfer rate||600MB/s|
|Maximum sustained data rate||138MB/s|
|Average rotational latency||4.2 ms|
|Random read seek time||8.5 ms|
|Random write seek time||9.5 ms|
|Spindle speed||7,200 RPM|
|Areal density||347 Gb/in²|
|Idle acoustics||2.8 bels|
|Seek acoustics||3.2 bels|
|Warranty length||Five years|
More importantly, according to Seagate's own data sheets, the Barracuda XT can only sustain transfer rates of up to 138MB/s. That's on the outer edge of the platter, which is the fastest part of the disk. Even short burst transfers from this edge top out at 169MB/s, the company says. The XT may be Seagate's fastest mechanical hard drive, but its peak media transfer rates are well within the capabilities of the second-gen, 300MB/s SATA interface.
"Surely the cache is fast enough," thou doth protest. You'd be right to assume that the XT's onboard DRAM memory could saturate a 600MB/s Serial ATA link, especially since Seagate itself demoed a prototype 6Gbps drive pushing nearly 590MB/s back in March. Except the XT's isn't nearly that fast. We asked Seagate for the XT's maximum cache burst speed and were told that it's only 301MB/s.
Seagate can certainly lay rightful claim to having the first SATA 6Gbps hard drive on the market. However, according to the company's own performance numbers, the underlying drive isn't fast enough to take proper advantage of the interface's faster data rate.
Your motherboard probably doesn't have any SATA 6Gbps ports, anyway, making much of this discussion academic. The 'cuda works just fine with 3Gbps SATA controllers, and this drive is more than just a test case for the 6Gbps standard. This is the crown jewel in Seagate's desktop hard drive lineup and the first four-disk implementation of the 500GB platters that debuted in the terabyte Barracuda 7200.12.
The Barracuda XT's platters have an areal density of 347 Gb/in², putting the drive at a bit of a disadvantage when compared with its only four-platter, two-terabyte counterpart, Western Digital's Caviar Black 2TB. The Caviar's 500GB platters can accommodate 400 gigabits per square inch, which is, well, more. Higher areal densities make more data available over shorter physical distances, helping to improve performance with sequential transfers. Of course, if we're talking sequential transfers, it makes more sense to reference linear densities, which we can derive with a little simple math. The 'cuda has a linear density of 18.6 Gb/in, while the Caviar packs in about 8% more bits, at 20 Gb/in. We're interested to see whether the XT can overcome that disparity in our transfer-rate tests.
Like the Caviar Black, the Barracuda XT ships with a five-year warranty. Seagate used to cover all its internal hard drive products for five years, but the longer term is now restricted to its premium and enterprise-class hard drives. The 2TB XT is currently the lone premium offering in Seagate's desktop stable, and there are no plans to extend the XT line to lower capacity points.