From mboxrd@z Thu Jan 1 00:00:00 1970 From: James Bottomley Subject: Re: bytes/CDB of SCSI pass thru grossly limited maybe Date: 28 Aug 2004 13:51:36 -0400 Sender: linux-scsi-owner@vger.kernel.org Message-ID: <1093715498.3682.4.camel@mulgrave> References: <20040828143124.GB2518@suse.de> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Return-path: Received: from stat16.steeleye.com ([209.192.50.48]:56461 "EHLO hancock.sc.steeleye.com") by vger.kernel.org with ESMTP id S266271AbUH1Rvu (ORCPT ); Sat, 28 Aug 2004 13:51:50 -0400 In-Reply-To: <20040828143124.GB2518@suse.de> List-Id: linux-scsi@vger.kernel.org To: Jens Axboe Cc: Alan Stern , Pat LaVarre , SCSI development list On Sat, 2004-08-28 at 10:31, Jens Axboe wrote: > > usb-storage, for example, where the protocol only limits us to 4 GB per > > transfer, there would be no problem allowing max_sectors to go as high as > > 65535? > > If the hardware and driver can handle it, there's no other limit. Actually, if you go through the SCSI stack, you're limited by our SG list slot allocation which we use a mempool for. The default maximum is 128, although there's an option to increase this to 256. These slots are the number of entries in the SG list we allow. If your driver disables clustering, and you havea no IOMMU, this limits you to 128* = 512kb on x86 bytes. Otherwise, it can go higher depending on the HW capabilities. Remember too that a lot of HW limits the number of SG slots (see the sg_tablesize entry for each driver). James