Just over four months on from our massive hard drive roundup (October 2002, page 74), and Serial ATA (SATA) hard disks have finally arrived. Phew. It's been an extremely long wait, but one that we were prepared to bide as SATA offers unparalleled (pun) features that are going to blow old Parallel ATA (PATA) disks away. On a performance level SATA doesn't seem to offer much over PATA at this stage, but more on that later.
It's the little things that go into a SATA drive, and its interface, that makes all the difference. And it's a huge difference. Under the hood, Serial ATA is about transmitting data serially, not in parallel like a standard PATA drive (the kind we've been using for well over a decade now). This means it sends less data per clock cycle compared to a parallel connection, although it runs at a higher speed. The reasons behind the push to switch to serial are fivefold: larger data bandwidth; more reliable transfers; lower power consumption; smaller cabling; and intercompatibility. SATA delivers on all of these fronts. Maxtor is behind a similar push to get ATA/133 widely adopted, but with the added benefits from SATA it will be fighting a losing battle.
Why We Need a New Architecture
We've been using PATA (ATA/33, ATA/66, ATA/100 etc) for over fifteen years, and while the theoretical data bandwidth has been steadily increasing there are serious problems with the fundamental PATA design.
Firstly, parallel IDE cables require more wires for signals and grounding. They are also highly vulnerable to interference, can only be 46cm in length, are wide and flat and take up a lot of room. Furthermore, with two drives sending data simultaneously over the one cable you can reach a bottleneck on the IDE channel.
If you've ever had to swap out a hard disk or optical drive, or tried to move disks around in a PC case, then you'll be extremely familiar with the headaches involved with the wide and stumpy parallel IDE cables.
Physically, SATA cabling couldn't be more different.
The 40-wire parallel cables are gone, replaced with just one small cable with six or so wires inside (four for data, and a couple for grounding). SATA is a one-to-one interface: one SATA port on a motherboard is capable of connecting to only one drive, so each drive has a single cable. The cables are so small that you can almost lay five in the width of one ATA/100 cable, which helps your PC circulate air and keep cool, as well as neatening the inside of your machine.
On the whole, a SATA drive is not much different from a PATA drive: it still uses the ATA protocols to transmit data, so a switch in drives is completely transparent to your operating system and applications. However, SATA is a packetised way of transmitting data which is based on Gigabit technology, with CRC checks on the packets and commands, opposed to PATA's parallel streams, and CRC checks on just the data. And while PATA transmits more data simultaneously, SATA transfers the sequential data packets faster.
One of the SerialATA Workgroup's goals was to reduce the signalling voltage of the hard disks, and it has done just that. While a parallel drive uses five volts a serial drive needs a mere 500 millivolts, which is a considerable drop. This means less power is required for a signal, there's less noise and interference and less heat generated.
Another major change with Serial ATA is that the old Master/Slave hierarchy is gone. Each and every SATA drive is a Master (drive 0) – there are no Slave drives. This change is transparent to applications, but for those legacy apps that require a Master/Slave configuration a controller chip, like those on a PCI SATA card, can route the incoming Master/Slave data to the relevant SATA drives. For the most part though, any SATA drive you use will be considered a Master drive.
The new cabling requires a new plug, and SATA plugs are small and can only be plugged in one way so you won't have problems with trying to plug it in the wrong way like you can have with EIDE.
Part of the Serial ATA design calls for two pins that rise slightly higher than the data pins on a SATA socket, which allows the drive to be grounded as it's being plugged in. Not only will this save you from frying a drive when you plug it in, but it also leads us to one of SATA's greatest features: it's hot swappable. You no longer need to power down a system to install a new drive, just plug it in and it works.
For system integrators or just for upgrading this is an indispensable feature. It's rumoured that external SATA drives will be shipping next year, even though Serial ATA was designed primarily as an internal interface.
The SATA power connectors are different too – they're much smaller than the traditional Molex plugs. But a word of warning: not all SATA drives will ship with the Molex-to-SATA adaptor cords that you need to get it running, so check with the shop before you make a purchase.
All of this ties in with the main goal of the SerialATA Workgroup, which is that SATA has to be transparent and 100% compatible with current PATA systems.
Serial ATA was also designed to be a future-proof technology with at least 10 years of life to it. There's a definite roadmap for improvements across the intervening years, with throughput increases from today's 150MB/s to 300MB/s in 2004 and 600MB/s in 2006.
It Lives!
The first (working) SATA drive we've been able to test in our Labs is the 120GB Barracuda IV from Seagate. In a demonstration of the disk at the PC Authority offices Seagate took great pains to point out to us that its disk is the first native SATA hard disk with the SATA controller on the hard drive, not a disk bridged from parallel ATA to SATA with an adapter. Whether or not this makes an appreciable difference in the performance is yet to be seen (we'll have to wait until the other drive manufacturers release their bridged SATA disks to test the theory), but it was more a show of Seagate's commitment to SATA than anything else.
In the middle of the presentation we decided get Seagate to hot plug a drive in, fresh out of the shrink wrap.
It was a very tense moment for everyone there considering that the drives were brand new and even the product managers hadn't really had a chance to test the technology themselves. To be honest, even I felt apprehensive and expected a bang and some sparks, but thankfully, the drive worked the first time as it was supposed to. Since then in Labs we've been hotswapping the drive like mad, and we haven't had a problem with it.
The Serial ATA Barracuda IV is a 7,200rpm drive with 120GB of storage and an 8MB cache. By Seagate's own admission the 8MB won't do much for the drive, but it's the way the industry is headed.
We tested the drive on a Gigabyte GA-8INXP motherboard that comes with two SATA ports, two IDE RAID ports and the usual dual IDE ports. This way we could benchmark the SATA hard disk against a conventional 120GB Barracuda ATA V parallel ATA drive, with DiskSpeed32 and Intel's IPEAK.
Overall SATA ups the theoretical transfer rate from ATA/133's 133MB/s to 150MB/s. This is a minor increase, and in our tests the difference was negligible. If you're looking for a huge performance increase you'll be disappointed, as the difference between the parallel and serial drive's performances was minimal. For instance, both drives scored within .001 of each other in IPEAK, but DiskSpeed32 told a more interesting story.
While the scores were extremely similar again, what did show up quite clearly was that the Serial ATA drive had much cleaner and more consistent data transfers