Dissecting Serial ATA
For most of the last 15 years, the most popular hard drive standard in existence for desktop workstations has been the ATA standard. Unless you haven’t cracked the case of a computer since before IBM shipped the first PS/2, you’ve probably dealt with an ATA-based device. As venerable and reliable as the ATA standard has been, however, there’s a new kid on the block--Serial ATA (SATA). Here’s what you’ll face with this new standard.
What is SATA?
Maxtor,
the patent holder for the ATA standard, has regularly improved the
standard to the point where ATA and Ultra ATA can now achieve rated
burst transfer speeds of 100 mbps and 133 mbps respectively. The term
ATA is interchangeable with the term IDE, and the ATA standard is also
sometimes referred to as Parallel ATA (PATA) because PATA transmits
data along its data path in a parallel manner.
However, because
PATA burst transfer rates are limited, it is fast becoming a bottleneck
in data transfer as drive manufacturers market products with
ever-higher data transfer rates and increasing drive speeds. SATA,
which in its earliest incarnation is already rated at 150 mbps, is set
to eclipse PATA’s burst transfer rates. Enhancements in SATA are not
just to be found in performance; they extend to items such as
scalability, easier installations, and better airflow. SATA is aimed
primarily at the high-end PC market and the low-end server market (or,
in other words, non-mission critical production servers, although this
may well change).
The Serial ATA Working Group is an industry
body that designs, develops, and delivers specifications for the SATA
interface. There are two parts to the working group. The first convened
in 2000 to set standards for SATA with respect to desktop applications.
The second was established in February 2002 to focus on the needs of
the server and network storage market segments.
To fully
appreciate the improvements SATA brings to storage technology, we need
to briefly examine the evolution and limitations of Parallel ATA. When
it first went to market, PATA was achieving transfer rates of 3.3 mbps,
and over the years this has gradually increased to transfer rates in
the 100 mbps range. There were other improvements over the years as
well, such as:
- The development of ATAPI to support other storage devices, such as tape drives and CD-ROMs
- Backwards compatibility to earlier ATA versions
- Cyclic redundancy checking for data integrity
- EIDE extensions for faster drive access
- Multiple transfer modes, including DMA and UDMA
As welcome as these improvements have been, the PATA interface nevertheless still suffers from some inherent design weaknesses, which are now coming to a head thanks to faster drive speeds and higher data transfer rates. There are three main limitations to PATA:
- Cable length
- Voltage requirements
- Data integrity issues
Cable length on PATA devices is limited to 18 inches (45 cm) because of signal attenuation. This is problematic in larger computer chassis with longer distances between connection points and can make certain physical drive configurations impossible to implement. Furthermore, the wide, 40-pin ribbon cables used to connect PATA devices are cumbersome and tend to restrict airflow inside a computer. This can lead to unwelcome hotspots inside a given machine. Because the cables are unwieldy, they are difficult to route; in any case, the length issue usually makes creative wiring impossible.
PATA devices require a 5-volt signal. The trend in chip design is towards smaller chips with lower voltages. The large chip pads needed to accommodate the 26 x 5-volt signals per ATA channel mean that, eventually, the chip pads would dominate the size of the chip. Higher voltages also mean that the machine’s overall power consumption is higher.
Data integrity in PATA does offer cyclic redundancy checks (CRC), but this does not extend to command data. SATA offers more in the way of end-to-end integrity of command data.
SATA benefits
Let’s move on to SATA and examine the benefits of this newly developed bus interface. Chief amongst these is the speed of data transfer. Earlier, I mentioned that SATA currently offers burst speeds up to 150 mbps. Well, by 2004/5 and 2007/8, SATA data transfer rates are set to go to 300 mbps and 600 mbps, respectively. Eventually, we may see burst transfer rates nearly 10 times faster than today’s PATA devices.
In comparison to PATA’s short, wide, bulky cables, cabling a SATA-based system is a dream. The serial cable is small and thin and can extend to one meter in length, making installation and cable routing in a big system case a doddle. The low pin-count makes for small connectors, so you can kiss those 40-pin ribbons goodbye. This cabling also improves airflow in the machine. Check out Figure A to see how small the cables are. Figures B and C show you close ups of the serial cable and the interface on a SATA drive.
Figure A: SATA cables are much more slender than traditional ATA cables.
Figure B: SATA devices have keyed connectors.
Figure C: Here’s the underside of a SATA hard drive.
In terms of voltage, SATA runs two data channels for sending and receiving, and 500 millivolts (mV) is all it takes to run SATA. This makes for better power consumption.
SATA is fully backwards compatible with PATA devices by way of multipurpose chipsets or the use of dongles for parallel-to-serial conversion and vice versa. This also goes for software, as SATA and PATA use the same drivers, and there is also no need to change or upgrade the operating system in any way. Figure D shows a dongle for PATA/SATA conversion.
Figure D: You can convert PATA to SATA through a dongle like this.
Serial ATA drives and adaptors use either “native” or “bridge” controller chips. The former offers maximum rated throughput (150 mbps) and the latter allows the conversion of a parallel-to-serial signal. The problem with bridge controllers is that they eat up bandwidth, and quite a lot of it: on a 150 mbps-rated device, a bridge controller may leave you with only 70-80 mbps bandwidth. This may not be a big issue for drive and adaptor manufacturers now, seeing as the very fastest IDE disks deliver 50-70 mbps bursts, but it will become more important in the medium term.
So how do you attach the drives to the rest of the system? At the moment, the prevalent method is to use SATA host adaptors, available in many different flavors from various manufacturers such as Promise, 3Ware, and Highpoint. These adaptors usually come with an IDE connector too, should you want to use a mixture of drives, not to mention the serial cables and often extra power cables. If you want, you can buy adaptors with 8 SATA channels for 8 separate drives! Some manufacturers offer bundles that include adaptor and drive(s). Shop around to see what you can find.
The market is beginning to see the first motherboard product offerings, which integrate SATA directly into the on-board chipset. By the second half of 2003, you should be seeing this as a standard feature. In Figure E, you can see the respective sizes of PATA (IDE) connectors and SATA connectors (highlighted in yellow)--clearly, there are major space advantages when using SATA cables.
Figure E: You can now buy motherboards with SATA connectors.
Another wonderful feature is that SATA does away with the need to set jumpers for master/slave drive configurations. Instead, when a drive is connected, the BIOS immediately recognizes it as a master drive numbered 1,2,3, and so on, depending on which SATA connector it’s hooked up to on the adaptor.
Average disk access time on an average SATA system running through an average 4-channel SATA host adaptor is 14.5 milliseconds. Disk read averages a maximum of 47 mbps and disk write averages a maximum of 36.6 mbps. Burst speeds average 79.4--perhaps not as high as one would expect given the theoretical figure of 150 mbps.
This is interesting because the average PATA drive manages 70-75 mbps bursts. That said, different SATA hardware combinations will yield higher burst speeds--and don’t forget the bandwidth performance penalty that is incurred when bridge controllers are used. The latter is really the crux of performance and, if you are intending to buy SATA hardware, then some in-depth research of different products is called for.
What about SCSI?
Some will understandably say that SCSI interface is forever king and others will say that SCSI’s days are now numbered. I tend to agree with the latter and here’s why:
- The Serial ATA II specification has had a smart command queuing capability added to it. This was previously a major attraction of SCSI; devices using the SCSI protocol are able to queue and execute requests without any assistance from the CPU or chipset.
- The fact that SATA devices don’t need ID numbers assigned, as do SCSI devices, makes them pretty much plug and play--another advantage.
- If you give SATA the same--and more--advanced capabilities as SCSI, then the price difference between the two may mean that SCSI will start to lose market share. But it is early yet and SCSI is extremely prevalent in high-end enterprise systems and likely will remain so for some time yet.
What’s it going to cost me?
Naturally, prices for SATA devices are going to vary. Ballpark figures for something like a Seagate Baracuda V 120 GB drive fall in the US$200 range. The same drive with an 80 GB capacity is around US$160. I mention Seagate’s drive because it appears to be a market leader in terms of pricing, and because Seagate has also developed SoftSonic, the name given to their new drive motor, which uses a fluid dynamic bearing. It is reputedly the quietest drive out there, and if you are using several drives (at home or in the office), then noise could become a concern.
SCSI drives usually average about US$100 or more, but you typically only get a capacity of between 18-36 GB, so buck-per-byte, they remain significantly more expensive. Remember also that SATA is new, so device prices haven’t started falling, as they surely will--at least a little. An 8-channel host adaptor from a company such as 3ware will cost in the region of US$550- US$580. A 4-channel adaptor will set you back about US$400. Clearly, putting a RAID 5 system together using Serial ATA is more cost-effective than going the SCSI way, but different needs and resources will dictate different solutions.
Another thing to bear in mind is that if you need "just a bunch of disks" (JBODs) for increased disk space in the form of network-attached storage (NAS) or direct-attached storage (DAS), SATA is probably your best bet, certainly in terms of price. There are NAS devices already on the market and some offer features such as hot-swap SATA drives, a big deal when things start to go wrong. The whole concept of SATA in this scenario just makes for a simpler implementation without the overheads imposed by SCSI and/or fiber channel.
If you want JBODs without the command and control systems (CPU, memory, basic OS) offered in NAS devices, you can also use SATA drives in disk boxes that connect directly to the system via SATA hubs. SCSI is touted as having the same capabilities, but the inevitable SCSI price tag may remain a hindrance.
The future is near
The PATA standard is in no danger of dying out just yet. The vast majority of PC systems (distinct from servers) run parallel ATA devices; this will be the case for some time still. If there is an upswing in PC purchasing, then the adoption of Serial ATA will be quicker. In terms of performance, it is fair to ask why one would need to upgrade now, given that benchmark performance testing has shown no great difference (yet) between the two technologies. This is a fair point and it may justify waiting until SATA becomes faster.
SATA does, however, offer benefits when it comes to power consumption, ease of installation, cabling, internal thermal dynamics, and drive configuration. These benefits alone may outweigh the speed issue, particularly for PC assemblers who will save time and money and experience less handling damage.