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

[Jens Axboe <axboe@suse.de>]

On Sun, Mar 14 2004, James Bottomley wrote:
> 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.

Ah, makes a lot more sense now, thanks :-)

-- 
Jens Axboe