From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jens Axboe Subject: Re: [PATCH] SCSI sym53c8xx_2: bigger transfer limits Date: Wed, 1 Mar 2006 17:11:18 +0100 Message-ID: <20060301161118.GF4816@suse.de> References: <20060301152929.GZ4816@suse.de> <1141227541.3276.26.camel@mulgrave.il.steeleye.com> <4405C538.4050709@torque.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Received: from ns.virtualhost.dk ([195.184.98.160]:12630 "EHLO virtualhost.dk") by vger.kernel.org with ESMTP id S1750962AbWCAQOv (ORCPT ); Wed, 1 Mar 2006 11:14:51 -0500 Content-Disposition: inline In-Reply-To: <4405C538.4050709@torque.net> Sender: linux-scsi-owner@vger.kernel.org List-Id: linux-scsi@vger.kernel.org To: Douglas Gilbert Cc: James Bottomley , Kai Makisara , matthew@wil.cx, linux-scsi@vger.kernel.org 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