T520 poor disk I/O thoughput

[Alun Liggins]

Hi
I have two T520's one of which has a reputation with users of being unresonably slow compared to the other. After checking with sar and glance I decided to run the following on each disk:
time dd if=/dev/dsk/cxtxd0 of=/dev/null bs=64k
for each of the disks on both servers.
On the second T520 the times showed a huge range from 10% to 10x slower, typically about half the speed of the other server. Both servers have simular usage.

Looking at the SEAGATE ST34371W disks the OK server reads these in 55-65 minutes. The other server is as follows:

controller c21/c22 85 minutes when quiet otherwise up to 10 times slower (900 minutes)
controller c23 75 minutes
controller c24 65 minutes

The layout of Bus Adapter's.
2/4 2/12 2/20 2/28
c6 c7 c21 c22

4/4 4/12 4/20
c9 c10 c23

8/4 8/12 8/20
c14 c15 c24

Both machines have 6 bus adaters with between 2 and 4 fast Wide SCSI cards on each. Each SCSI has between 6 and 10 disks attached. 130 disks in total.

Anybody help me with this?

[Vincenzo Restuccia]

Verify with top the load software in both hosts.

[Edward Sedgemore]

Youve done some dd tests on the disk, good. Are they the same model disks on both servers ?

How many disks do you have on each scsi controller ? the more the slower.

Whats the overall system wio% (Wait on I/O%) ? use sar 1 100 to get an average (during normal working hours). Are the 2 system similar in their wio% ? If so then the so called slower performance from one of them may be nothing to do with I/O. (have to look at application setup, RAM, etc.)

[Paula J Frazer-Campbell]

Hi Alun
Whilst the you have targeted the disks to measure server speed the problem may lie somewhere elso on the slower server and this will slow the disk test down.

Check the server using tools like glance, swapinfo, sar, iostat etc.
Also look at the kernel config on each server as you say they are dong similar jobs with similar applications then the kernels should be similar.

I would be inclided to believe that you slow disk test are a symptom rather that causitive.

HTH

Paula