Promise SX8 performance issues and CARM_MAX_Q

[Bernard Blackham <bernard@blackham.com.au>]

Hi,

Playing with a recently acquired Promise SX8 card, we've found
similar performance results to Matt's post to lkml a few months back
at http://marc.theaimsgroup.com/?l=linux-kernel&m=110175890323356&w=2

It appears that the driver is only submitting one command at a time
per port, which is at least one cause of the slowdowns. By raising
CARM_MAX_Q from 1 to 3 in drivers/block/sx8.c (it was 3 in an
earlier pre-merge incarnation of carmel.c), we're getting very
notable speed improvements, with no side effects just yet.

Knowing very little about what this change has actually done, I've a
few questions: 

 - Should this be considered dangerous?
 - Why was it taken from 3 to 1?
 - Is CARM_MAX_Q a number defined (or limited) by the hardware?

Thanks in advance,

Bernard.

-- 
 Bernard Blackham <bernard at blackham dot com dot au>
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