[Beowulf] StorageTek 6140: slow speed

Jörg Saßmannshausen j.sassmannshausen at ucl.ac.uk
Thu Nov 13 02:58:05 PST 2014

Dear all,

some time ago I have acquired a StorageTek 6140 more or less for free. 
As one of the PIs here needed large disc space, I decided to use that as this 
was the only thing I had at the time. I have set up the 16 discs so that I got 
three sets of RAID5 discs with 5 discs each and on top of that I put a LVM2. 
Thus I got one spare disc which should act as a global hot spare one. 
The aim was to make sure there is the most data security and the device does 
not support RAID6. 

However, the disc speed is appallingly bad. For example, if I am doing this:

$ dd bs=1M count=1024 if=/dev/zero of=test oflag=dsync

I get that speed when I am writing straight to the disc array:

1073741824 bytes (1.1 GB) copied, 124.002 s, 8.7 MB/s

For comparison, writing to the disc where the OS is resting on the same box I 
get that speed:
1073741824 bytes (1.1 GB) copied, 50.6415 s, 21.2 MB/s

and writing to array at the front end of the cluster (different from above) I 
get this speed:
1073741824 bytes (1.1 GB) copied, 3.24406 s, 331 MB/s

Clearly, there is a problem. 

The front end of the SUN disc array is a 32 bit Xeon(TM) CPU 3.06GHz machine. 
I would have thought that is ok, given that all it does is shovelling data 
from the network device down to the fibre optics.

I was wondering, could it be that my setup with the 3 RAID5 is the problem? 
Any suggestions?

The users are not happy and I can see why. I would like to continue using it 
as a new box costs money which is obviously not around right now.

All the best from a mild London


Dr. Jörg Saßmannshausen, MRSC
University College London
Department of Chemistry
Gordon Street

email: j.sassmannshausen at ucl.ac.uk
web: http://sassy.formativ.net

Please avoid sending me Word or PowerPoint attachments.
See http://www.gnu.org/philosophy/no-word-attachments.html
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