Re: [PATCH] SCSI sym53c8xx_2: bigger transfer limits

[Jens Axboe <axboe@suse.de>]

On Wed, Mar 01 2006, Douglas Gilbert wrote:
> James Bottomley wrote:
> > On Wed, 2006-03-01 at 16:29 +0100, Jens Axboe wrote:
> > 
> >>Strictly speaking, the clustering bit is unrelated. I seem to recall
> >>Gerard years ago talking about some sym chips that did not like
> >>clustering, hence it was disabled.
> > 
> > 
> > Yes, I remember that too ... I've never been able to find out which
> > chip, though ... the scripts all seem happily coded for variable size sg
> > segments.
> > 
> > However, given the new way 2.6 does memory allocations,
> > ENABLE_CLUSTERING will probably make quite a difference to the size of
> > the sg list ... since we try to allocate contiguous pages, physical
> > merging becomes much more of a possibility (I think I last measured it
> > at around 30% of all SG tables, as opposed to <1% with the old
> > allocation method).
> 
> James,
> So the maximum data carrying size of a scatter gather list
> is not deterministic? Is the worst case (page_size * SG_ALL)?

It's block layer restricted (well not because of the block layer, but
the limits that the hardware tolds us that it has), typically page_size
* max_sg_entries is the correct answer but it may of course be more.

-- 
Jens Axboe