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Linux Tutorial - The Computer Itself - Hard Disks - Serial ATA
  RAID ---- Floppy Drives  


Serial_ATA.html


by James Pyles

Serial ATA (Advanced Technology Attachment), drives began arriving on retail shelves in November of 2002. They use a completely new interface between the hard drive and the motherboard and are quite likely to replace the standard ATA interface in the next few years.

So just what is the big deal about Serial ATA drives?

To answer that, we need to delve into a bit of history. Since 1989, IDE drives have been the standard hard drive interface. Except for enlarging the pipeline from 33 to 100/133 MB/sec with the invention of ATA in 1998, nothing has changed about the interface between the HDD and the motherboard. For almost 15 years (an eternity in the world of computer technology), the standard connection between the hard drive and the motherboard in PCs has been the same parallel interface.

So how is a Serial ATA drive different?

Serial ATA or SATA drives are a new standard in HDD/motherboard interfaces and are touted as the next mainstream storage interface. SATA drives are fast with the first generation having a volume of 150 MB/sec compared to Parallel ATA (PATA) drive's 100 MB/sec. Generation two (coming out in 2005) is predicted to go up to 300 MB/sec. This is also better than USB interfaces at 60 MB/sec and generation two will be better than current firewire volume of 200 MB/sec.

The current ATA drive's parallel interface will eventually become a throughput bottleneck, especially as drive densities continue to increase. In 2001, major vendors were able to put 40 GB of storage on a hard drive per platter. By the end of 2003, it will likely be 80 GB and future densities are predicted at 120 GB per platter. This impacts throughput because the more density, the more pressure to push data faster across the interface.

SATA interfaces can make use of cables twice as long as IDE cabling at a full one meter. The cable is thinner, .25 inches compared to a two inch IDE ribbon, and plugs into the SATA drive in a manner similar to a network cable.

SATA drives are also "hot swappable" meaning no more reboots, they are quiet (thanks to fluid bearings), but not quite silent, use less power overall, and come with either a 2 or 8 MB cache.

Now for the downside

Currently, SATA drives have not yet entered the mainstream PC market. This is predicted to happen by 2004 when it is believed that major vendors like Dell and HP will start offering SATA drives in their upper end systems. If you want to use a SATA drive in your PC, you will not only have to purchase the drive, power supply and cable, but a PCI SATA adapter card. Your motherboard must also be SATA capable. For SATA to enter the mainstream market, Intel will need to integrate this standard into their chipset…planned by the middle or end of 2003.

The other downside: only one device can be used per port. This means that there is no RAID capacity for generation one SATA since you can't run multiple drives on the same channel.

SATA II

That's right, there's more. The SATA working group announced that SATA II will allow additional ports to support up to 15 drives. This will benefit mostly servers running multiple rack-mounted drives.

Other extensions announced by the group are native command queuing, giving hard drives the ability to prioritize data requests from multiple CPUs, doubling the bandwidth from 1.5 to 3 GB/sec, fallover switches for when high reliability is vital, and connections for external SATA II drives.

The speed and increased throughput of SATA drives are extremely important in the future of servers. While an end user is unlikely to notice the performance difference, the speed improvement will be important in the use of RAID to operate multiple drives.

Yeah, but how much does it cost?

I found a comparison of Maxtor SATA and PATA drives. ExtremeTech published a comparison between Maxtor's DiamondMax Plus 9 PATA and the Plus 9 SATA. Each drive has a formatted capacity of 200 GB, 66.67 per platter and a rotational speed of 7200 RPM. The PATA drive will set you back $210 USD while the equivalent SATA drive is a bit more at $275. Maxtor's SATA FAQ states that SATA drives will be typically a bit more than equivalent PATAs, throughout 2003 but as SATA drives become more common, the prices will reach parity. The SATA Plus 9 currently comes in capacities of 60GB, 80GB, 120GB, 160GB, and 200GB.

Not trying to favor Maxtor, I went shopping at Atacom.com for comparable Seagate drives and looked at the Seagate PATA 80GB ST380021A versus the SATA Barracuda V 80GB. The PATA drive costs $83.95 while the equivalent Barracuda ran $125.95.

What about drivers?

According to serialata.org, SATA supports legacy drivers for PATA drives. In other words, you can install a SATA drive on a motherboard with the appropriate PCI SATA adapter and use the OEM's existing parallel ATA drivers. Vendors will begin supplying bridges for parallel to serial conversion for legacy devices. This being said, Seagate's support page recommends that, if your operating system (Windows 2000/XP in this case) doesn't detect the SATA drive during installation, you may need to go to the vendor's website to acquire additional drivers.

Although many newer motherboards have SATA controllers, not all do and obviously older motherboards do not support SATA. If you have a motherboard that does not support SATA, all is not lost. You can buy extra SATA controller cards for between $20-$30 (US).

Silicon Image is providing parallel to serial conversion for Linux. Their drivers are supplied by the Linux ATA development site.

SATA and Linux

In June 2003, Pogo Linux included three new Linux servers using Seagate SATA drives in its StorageWare line. They claim that these servers are the first to use SATA for Linux. The StorageWare S140, S212, and S316 are all powered by Intel Xeon processors with Hyper-Threading Technology.

The SATA drives are configured with a 3Ware 8500 Escalade SATA RAID controller in a RAID 5 array. According to Pogo Linux, this will enable the StorageWare servers to give high speed performance at a fraction of the cost of even the latest, high-end SCSI based servers. You might want to save up your pennies if you're considering a purchase as the prices are $3,499, $7,499, and 9,499 respectively for the above-listed servers. Pogo Linux also produces the Velocity-DXWorkstation which comes with SATA, IDE, or SCSI starting at $1,979.

None of this means you actually have to spend thousands of dollars to use SATA with Linux. You will, at this point, have to buy the drive, connectors, power supply and adapter and perhaps download the drivers. For a few hundred dollars, you'll have all the materials to enter the world of Serial ATA. Keep in mind that SATA drives are just now becoming available in mass quantities and you will still have to wait until PC OEMs come with SATA integrated. I mentioned above that this is predicted for 2004 but the Maxtor SATA FAQ page was particularly evasive. It's likely that negotiations between the major relevant vendors are, as of this writing, still in progress.

If you plan to use SATA and Linux on a home or small office system, you will likely have to play around with the installation and configuration. As with anything new, there is uncharted territory to explore.

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Copyright 2003 by James Pyles. Licensed under GNU General Public License (Used with permission of the author). See here for details. All rights reserved.
  




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