What is Serial ATA
The first ATA hard drives appeared on the scene in the late 1980s, and have evolved into the latest ATA100 and ATA133 standards. Also known as IDE (for integrated drive electronics), the ATA standard has gone through a number of iterations. Todays ATA hard drives max out at 133MB/sec (Maxtor) and 100MB/sec (everyone else). The original ATA standard specified a connection speed of 3.3MB/sec. Early ATA drives offered 10-40MB of storage — a staggering amount at the time, but completely useless for most PC applications today. Capacities have evolved along with connection speeds, and we now have 320GB ATA drives available. However, todays hard drives still use an interconnect standard thats over fifteen years old, even as capacities and drive technologies have progressed.
The ATA standard is a 16-bit, parallel connection. Parallel ATA uses source-synchronous (non-interlocked) clocking, which means that the clock signal is actually sent with the data. This can create problems as data rates — and hence, clock rates — increase. Because of potential signal reflection and signal skew issues, the ATA100 standard reduced the voltage for ATA100 signaling to 3.3v. The high clock rates also require 80-conductor cables, with alternating ground and signal wires. The net result is a maximum cable length of 18 inches for reliable operation in a wide variety of environments.
Serial ATA Defined: Serial ATA is, as the name implies, a serial link. A single Serial ATA (S-ATA) cable consists of a minimum of four wires, with differential pairs for transmitting and receiving data. The standard also allows for additional ground wires as deemed necessary. Maximum cable length for the S-ATA 1.0 standard is 1 meter (roughly 3.1 feet). This makes external S-ATA drives possible.
S-ATA is also point-to-point. Each S-ATA connection supports a single drive, so the days of figuring out which jumper to set for master or slave will become an historic artifact.
Making S-ATA point-to-point also makes termination much easier, as opposed to parallel ATAs requirement to have a device attached to the middle of the cable. Todays systems typically only support two S-ATA connections. This is partly because current systems still require parallel ATA connections and partly because all of todays Serial ATA implementations work through PCI host adapter cards or chips. Being bound to PCI adds additional overhead and potentially limits throughput.
S-ATA also offers “first party” DMA support, meaning that devices arent dependent on a host controller for DMA. The standard also has hot-swapping designed in, which means you can (in theory) swap drives while the system is running.
S-ATA uses a 7-pin connector (to accommodate any additional ground wires), and is considerably more compact than the parallel ATA plug.
As you can see, four S-ATA cables and connectors take up roughly the same room as a single parallel ATA cable. In the future, when motherboard core logic directly supports S-ATA, well probably see as many as four S-ATA connections on a motherboard.
Unfortunately, parallel ATA wont vanish overnight. If nothing else, optical drive makers will transition to S-ATA more slowly, since they view the additional bandwidth as more a luxury than a necessity for their applications.
Performance and Compatibility
Serial ATA had two key goals beyond improved signaling integrity:
- Better performance: S-ATA achieves improved raw performance by increasing the data rate substantially. The clock rate of Serial ATA is 1.5GHz. The data is encoded with 8b/10b encoding, which reduces the potential bandwidth by 20%. This yields a net bandwidth of 150 megabytes per second of actual data.
- Software compatibility with parallel ATA: At first blush, it might seem like the operating system will just recognize the drive when you plug it in. In reality, youll still have to install drivers to achieve proper operation. Software compatibility means that the internal protocols are still compatible with parallel ATA, so that software thats dependent on a standard ATA connection (e.g., backup software) should still work. We verified this in our testing. We were able to back up a Serial ATA partition from the DOS prompt using Drive Image 5 with no problems.
The issue of performance is a complex one, due to the fact that the PCI bus lies between todays S-ATA host adapters and the rest of the system. If there is no contention on the PCI bus, then the maximum data rate theoretically possible is 133MB/sec. In practice, this is usually less — more like 85-90MB/sec on Intel-based systems and as low as 40-50MB/sec in systems using older (pre-8233) Via South Bridge chips.
However, most hard drives — even fast, high-density, 7200RPM drives — cant push data out at 150MB/sec. In practice, the best single hard drive data rates for ATA drives max out at around 44MB/sec sustained, from the outer tracks. Substantial PCI traffic from other sources, such as NICs or sound cards can adversely impact the overall PCI throughput. The picture gets worse once you move to RAID devices.
What this means is that were unlikely to get a true picture of Serial ATA performance until we have core logic that places the S-ATA controller north of the PCI bus.
Nevertheless, there are a large number of motherboards already shipping with motherboard-down PCI S-ATA host adapters. Serial ATA drives will also be attractive because you can load up additional drives without affecting the number of parallel ATA connections.
Want to see how one of the first Serial ATA drives actually performed? Check out our companion story, a first look of the Seagate ST3120023AS.