Red Hat Bugzilla – Bug 139551
FC disk read performance ~50% less than write performance
Last modified: 2007-11-30 17:07:05 EST
Description of problem:
Comparing the Fibre Channel read/write performance to a direct
attached JBOD disk, seeing decreased read performance on EL3.0 as
compared to AS2.1. Seeing the same results if using a QLogic or
Emulex HBA. So I can only assume that this is a Red Hat issue.
Read dd if=/FCdisk1/data of=/dev/null bs=100k 40-41MB
Write dd if=/data of=/FCdisk1/data1 bs=100k 0-15MB
Read dd if=/FCdisk1/data of=/dev/null bs=100k 19-20MB
Write dd if=/data of=/FCdisk1/data1 bs=100k 34-35MB
Notice that the read performance on AS3.0 is about half the write
performace. I get similar performance results if using Bonnie. I only
mention that because with the dd command, it only allows or does one
outstanding request at a time.
Several of our customers have also seen this problem and we direct
them to Red Hat.
Can anyone explain why this is occurring and if there are some
configuration settings, or patch to increase or make the performance
number more consistent?
Version-Release number of selected component (if applicable):
Steps to Reproduce:
Which RHEL 3 kernel version are you using?
There are some performance improvements in Update 4. U4 is in beta
now, and is due out in December. Please re-test with U4.
I'll add Update 4 and retest. I've never done an update before, where
do I find it since U4 is still in beta?
Instructions sent by email.
Randy reports that he can not reproduce this problem on U3:
"I re-installed AS 3.0 on the same system and still not seeing the
read performance issues and I had previously. Getting about the same
read (50MB/sec) and write (54MB/sec) performance."
He has not tried U4.
Closing this BZ. Reopen if the problem reoccurs.