Re: bytes/CDB of SCSI pass thru grossly limited maybe

[Jens Axboe <axboe@suse.de>]

On Sat, Aug 28 2004, James Bottomley wrote:
> On Sat, 2004-08-28 at 13:55, Jens Axboe wrote:
> > Probably bio and SCSI should use a unified MAX_PAGES define, there's not
> > much point in bio setting up a 256 page bio_io_vec slab and mempool, if
> > noone can use it.
> 
> Sure, ours is SCSI_MAX_PHYS_SEGMENTS ... by default, we just use
> MAX_PHYS_SEGMENTS from the bio headers for this, which is 128, we could
> set this to BIO_MAX_PAGES, sure.
> 
> We allow values of 32,64,128 and 256 (the smaller ones were for tiny
> systems with SCSI stacks), the largest one for SGI/Qlogic.
> 
> However, I did ask the IBM spearker at OLS who was doing the elevator
> analysis to retry with 256 instead of 128, and he reported no difference
> within error, so I don't think there's much evidence that 256 actually
> does anything useful (other than consume a bit of spare memory).

Since SCSI doesn't build > 128 pages anyways, yes it doesn't make sense
to maintain a BIO_MAX_PAGES of 256. Didn't the SCSI part used to be 256
pages as well, I'm pretty sure that's what I put in when the
scsi_malloc() crud was dumped?

bio has 1, 4, 16, 64, 128, 256 pools. 32 might make more sense, I seem
to recall mpages using that. I'll see if I can sneak a BIO_MAX_PAGES
reduction in, and spend that extra pool on 32 instead :)

-- 
Jens Axboe