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

[Joshua Baker-LePain]

On Wed, 18 Jun 2008 at 8:24pm, Joe Landman wrote

> David Mathog wrote:
>> 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.

ISTR hearing some rumblings at some point about some LTO tape drives not 
liking Adapted SCSI.  Mine run on LSI boards, and run quite nicely.

>> 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? 
>
> Have you adjusted the block size up?  maybe try
>
> dd if=/dev/zero of=/dev/nst0 bs=8M count=1000

A very good suggestion.  In my testing, 32KB blocks (the default for 
amanda) yielded only about 27MB/s (in line with your 8KB blocks above) 
while 2M blocks got me to 60MB/s (all testing done with tar and a very 
fast disk system).  Note that I couldn't go much bigger than 2M or I 
started getting errors like "/dev/nst1: Cannot write: Value too large for 
defined data type".

> this is about 8 GB, so it hopefully shouldn't take too long.  At 21 MB/s, 
> this is about 50 seconds per gigabyte, so if this takes 400 seconds, you may 
> have a problem.
>
> This said, the tape drive numbers we see quoted are "best case" scenarios, 
> with optimal block sizes, a strong wind at the tapes back, and no monopole 
> flux nearby.  Burning incense may or may not help the speed, YMMV...

In addition, 2x hardware compression is almost always a lovely fairly 
tale with absolutely no basis in reality.  That said, the LTO hardware 
compression is rather nice, in that it recognizes uncompressible data and 
doesn't try to further compress it.

And, as always, be sure you used the proper color goat in your ceremonies.

-- 
Joshua Baker-LePain
QB3 Shared Cluster Sysadmin
UCSF