Disk read performance problem with PE2650 and Redhat AdvancedServer products.....

Gary Mansell Gary.Mansell at ricardo.com
Fri Oct 10 08:41:00 CDT 2003


Dear all,

I have a Dell PE2650 with 2x 2.4GHz Zeons (Hyperthreading enabled), 2GB
RAM, 2x18GB internal system disks in a HW mirror configuration using the
PERC 3Di, The external RAID unit is an EMC FC4500 with 9x73GB disks in a
RAID 5 configuration. The RAID unit is connected using a QLogic QLA2310
FCAL card.

When I originally bought the machine I found a read performance problem
with RH AS 2.1 (fully up2dated) and had to install RH Linux 7.3 which
did not seem to have the same problem. I found that reads from both my
external RAID unit and the internal PERC mirrored system disks were
awfully slow - about three times slower than with Redhat Linux. There
was definitely something wrong somewhere as reads were slower than
writes to the same device!

The time has now come to add another 1TB of disk to the array and at the
same time I would like to upgrade the system to RH AS 3.0. I have built
a spare PE2650 with RH AS 3.0b2 and have found that the disk read
performance is still a problem with the internal PERC mirrored system
disk. I have not been able to test it against the external FC4500 as it
is used continually on the production system.

I have tried further tests in an effort to isolate the cause:

1) I have installed RH 9.0 on the spare PE 2650 described above and this
does not exhibit the read performance problem.
2) I have built a desktop Viglen PC with RH AS 3.0b2 and this does not
exhibit the read performance problem with either it's IDE system disk or
an externally attached SCSI disk or an externally attached Software RAID
0 set.
3) I have built the PE 2650 with RH AS 2.1 and see the read performance
problem with the internal PERC mirror but not with an external single
SCSI disk or an external Software RAID 0 set.

On the surface, it would appear that RH AS products have trouble reading
from HW RAID devices whereas the community versions of RH Linux do not.
This makes no sense to me though as the PERC system mirror is just
presented as a SCSI disk to the O/S via the aacraid module and the
external HW RAID 5 device is presented as a SCSI disk to the O/S via the
qla2300 module.

Any suggestions as to what the problem could be would be gladly
appreciated as I am starting to pull my hair out here now.


-- 

Gary Mansell
Technical Computing Team Leader
IT Department

Ricardo Consulting Engineers Ltd.
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