USB becoming universal slow bus

USB is too slow in these days of 500GB capacity serial ATA (SATA) disk drives. With disk-to-disk (D2D) backup becoming increasingly popular more and more data is being pumped across USB cables to external disk drives. It is simple and convenient and thus attractive to workgroups, departments, small and medium business and small office./home office server users.

There is no need to battle with stored tape cartridges, searching for the right one and trying to work out how to restore from your backup software package. Restoring a file should, ideally, be like accessing any other file on a system - using a drag-and-drop GUI.

But the universal serial bus is slow. It operates at 480Mbit/s but transfers data at a slower speed than that. This is due to a protocol conversion overhead. Assume an external ATA disk connected by USB. The disk's ATA command set interface has to be converted to/from USB as data travels to/from the disk and then re-converted at the other end inside the server/PC.

One effect of this is the burst transfer rate. It is 33.5MB/sec with USB but 111.3MB/sec with SATA 1, the 1.5 Gbit/s first version of the SATA protocol. The FireWire (1394) external interface is better than USB at burst transfers, measuring 36.2MB/sec even though its raw interface speed is slower at 400MBit/s.

Transferring a 20GB backup file to an external USB-connect SATA drive would take more than ten minutes whereas a SATA transfer would take just a little under six minutes. With a large backup of, say 100GB, the timings would be under an hour with SATA and heading towards two hours with USB.

Externalising SATA

Last year an external SATA (eSATA) standard became known (and here).

There is an external SATA standard. It was defined in 2004, some little time ago, and specifies the cabling, connectors and signal requirements needed. Silicon Image has a white paper describing eSATA and it is available here.

An ATA disk can pump data out faster than USB can accept and deliver it. So it obviously makes sense to have an external ATA disk interface that goes as fast as the disk. The eSATA interface does that but requires that external disk drives have an eSATA socket and also that the host PC or server also has an eSATA socket.

The two are connected by an eSATA shielded cable up to 2m in length.

eSATA products

La Cie has produced an eSATA drive product, the LaCie Two Big eSATA & USB. It is a 2-disk RAID array with hot-swap drives. The eSATA spec supports hot swappability whereas USB does not, although USB is plug-and-play it could best be characterised as cold swap.

There are two versions of the La Cie product: a 500GB one; and a 1TB one. La Cie also sells PCI-X and PCI Express SATA II 3Gbit/s cards to insert into the host PC or server and provide the external eSATA socket needed for the cable.

Unlike USB where power can be delivered to the external device through the USB data cable, eSATA devices need a separate power supply.

Seagate has also produced an eSATA product. This is a 300 or 500GB single external drive, over-viewed here. An eSATA RAID Controller can be used to group two or more of these drives together to provide RAID facilities.

Seagate is positioning these 7,200rpm drives as online drives for data-intensive users such as digital artists, but, clearly, they can be used for backup just as well.

Iomega has a 320GB eSATA desktop drive. Iomega reckons it's good for video editing, the digital artist market, and lightning-fast backups.

Western Digital is expected to release an eSATA drive this year.

eSATA PCS and servers

There are very few eSATA PCs as yet, that is, PCs or servers with built-in eSATA connectors.

Intel has a motherboard chip, the ICH8 'Southbridge', which includes eSATA ports.

Dell has produced a high-end laptop, the XPS M2010, with an eSATA port. This suggests that it is following the Seagate line that eSATA is for digital artists and other data-intensive workers who need access to fast and high-capacity external storage. D2D backup is not part of the marketing equation here.

A list of eSATA products can be found here. It lists no PCs or servers though. For the time being implementing eSATA drives and drive arrays is practically a job for the DIY sysadmin or a reseller. We might find, we should find that, during 2007, SOHO and SME servers and PCS start appearing with eSATA connectors built-in, from mainstream suppliers such as Dell, HP and IBM.

The USB was right on the money when it appeared, riding and helping to drive the move to serial interfaces. Serial ATA, like serial SCSI, is now becoming mainstream and USB is starting to be left behind. Unless it is given a radical speed jump its name will become a misnomer, as it is going to become the none-universal serial bus interface - NUSB.


There is more on eSATA on Wikipedia. Naturally the usual Wikipedia caveats apply as to the credibility of the text originators.

This story, "USB becoming universal slow bus" was originally published by


Copyright © 2007 IDG Communications, Inc.

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