On Fri, 14 Nov 2008, Simen Stavdal wrote:
Hi Lars,
The USB 2.0 Specification says max 480Mbps, and is to be considered a
theroretical max.
This equates to about 60MBytes/second.
The devices that connect through the bus rarely get even close to this
rate.
In fact, if you compare it to the SATA-2 specification says 3000Mbps
(375MBytes/second, roughly 6 times faster than USB 2.0).
Disks on SATA-2 Controllers sometimes achieve as much as peak of 100MB/s
write performance which equates to about 800Mbps,
(and this is for rather fast disks, the average write speed is somewhere
around 65MBytes/second when directly connected to SATA-2).
This shows that the specification is a lot higher than the actual
throughput you get.
Your external disk is connected through a USB 2.0 Connector, and the disk
itself is connected through a controller (probably PATA or SATA) in the
disk housing.
There are many bottlenecks between your soekrisbox and the actual disk,
limiting the transfer rate quite a lot.
1. The soekrisbox itself, and it's USB 2.0 controller.
2. The Adaptor in the disk enclosure (from USB2.0 to SATA/PATA).
3. The disk itself (could be limited by a number of things, such as the
rotational speed of the disk etc).
4. Never believe marketing! They never show the real performance, only
the thoretical, not limited by all the bottlenecks.
I would say that your speed of 5,3MBytes/second, is not to far off what
to expect. You could obviously test this on another machine to find out
*where* the bottleneck is.
If you get substancially higher speeds, then you know it is the
controller in your soekrisbox...
I am always getting similar transfer speeds (up to 5MB/s) under OpenBSD
(and the same with NetBSD) with external USB hard disks too, while the
real transfer speed under some other OS's (Linux, Windows) is around 28
MB/s on the same hardware. I mean, on the very same pieces of hardware,
usually running a different OS from a live CD. As far as I remember, it is
for both reading and writing.
It seems to me to be by design of umass(4) or other USB drivers. There
must be reason for it.
Regards,
David
