Time-honoured problem : RHEL3 iowait performance

[Robin Humble]

Hi Brendan,

On Fri, Mar 02, 2007 at 12:52:14PM +0000, Brendan Heading wrote:
>Robin Humble writes: 

BTW, I forgot to mention that I'm using CentOS4.4 x86_64 which could
have very different to characteristics to RHEL3.

>>when I set the read-ahead on the lvm to the same value as the non-lvm it
>>goes at very close to the same speed. I find the below to be a good
>>(large file) value for both:
>>  blockdev --setra 16384 <block dev>
>Thanks Robin. LVM actually reports a readahead of 1024, where my raw disks 
>are set to 120. 

RA was set to 256 on md raid0 and raid5 lvm partitions. argh.

>I have vm-readahead set to min 1024 max 2048 in the sysctl... 

hmmm.. I haven't tried vm-readahead.

>hardware. The "fun" associated with RAID5's characteristics may effect LVM 
>in strange ways though .. 

+/- setra, software raid seems happy enough, but yeah, hardware raid +
their kernel drivers may do other odd things.

cheers,
robin