Re: PATCH [5/15] qla2xxx: SG tablesize update

[James Bottomley <James.Bottomley@steeleye.com>]

On Sun, 2004-03-14 at 09:51, Jens Axboe wrote:
> The explanation of why this was done (request queue limitations, huh?)
> sounded pretty bogus as well.

Actually, to be fair, I think he means the qlogic request queue, not the
block one.

The Qla chips are rather weird in that they have a single issue queue
(whose size you can vary) but whose entry formats are fixed.  If I
remember correctly, an initial command can have 4 SG elements, but a
follow on entry can have 7 (not sure of the figures).  But anyway, large
SG commands end up having to find multiple entries in this queue (and
being a single issue queue for the entire card, it has to be mutexed
while you search).  The more resources you need, the more difficult the
search and the more contention you generate on the resource mutex.

I talked to Arjan a bit about this type of card and how best to handle
it in our current infrastructure, but I haven't got any further than
thinking about it.

James