[Beowulf] OT: LTO Ultrium (3) throughput?

[David Mathog]

If any of you have an LTO Ultrium-3 drive what kind of speeds are you
observing?  On one Linux system here (kernel 2.6.24-19) we have an
HP Ultrium-3 attached to an Adaptec ASC-29320ALP U320 controller.
There is nothing else on that SCSI bus, termination and cable seem good.
Getting into scsi-select from the BIOS shows everything set to 320.
No error messages or warnings are appearing. Yet:

  dd if=/dev/zero of=/dev/nst0 bs=8192 count=10000

only moves 21.3MB/sec.  The HP documentation indicates 432GB/hour,
(compressed) which is 120 Mb/sec, so we're off by 6X (or maybe 3X for
2:1 compression, either way, a lot).  The system's CPU and memory
aren't rate limiting as

  dd if=/dev/zero of=/dev/null bs=8192 count=10000

moves 6.9GB/sec.  

Any thoughts where the bottleneck might be? 

Thanks,

David Mathog
mathog at caltech.edu
Manager, Sequence Analysis Facility, Biology Division, Caltech