Description of problem:

I am grappling with IO bottleneck issues on Xeon servers 
running RHEL3. They seem to max out IO capacity at 
30-40 MB/s. Some say that's expected, but no normal
ranges are anywere to be found. Still I think there is 
room for improvement.

We do 2 Gb/s FibreChannel to an EMC Symmetrix and I 
have the EMC engineer make statistics of the EMC device
while I do a simple read test with

dd bs=65536 if=/dev/sda1 of=/dev/null

I also alter vm.max-readahead in two test to be 1023
and 127 respectively.

I expect to see the read sizes on the EMC to change. 

- I expect them to be 64 kB minimum, because I set
  read buffer size to 65536 on dd.

- I expect them to be 64 kB when vm.max-readahead is
  127.

- I expect them to grow up to 512 kB when I set vm.max-
  readahead to 1023

Yet, in all cases the EMC gets read requests of 32 kB
only. 

Question:

- is there a hard limit in the linux kernel for IO buffers?

- is there a hard limit in the QLogic 2200 driver?

- is there a limit on the QLogic card itself?

- is there a sysctl parameter that I can set to crank the
  maximum transfer size up to at least 128 kB?

Version-Release number of selected component (if applicable):

kernel: 2.4.21-15.0.3.ELsmp

QLA driver version 6.07.02-RH2

How reproducible:
Steps to Reproduce:

see above

Actual results:

32kB transfer sizes on the SAN disk array.

Expected results:

64kB and 512kB transfer sizes on the SAN disk array.