From owner-freebsd-current Tue Dec 24 17:08:24 1996 Return-Path: Received: (from root@localhost) by freefall.freebsd.org (8.8.4/8.8.4) id RAA19454 for current-outgoing; Tue, 24 Dec 1996 17:08:24 -0800 (PST) Received: from dyson.iquest.net (dyson.iquest.net [198.70.144.127]) by freefall.freebsd.org (8.8.4/8.8.4) with ESMTP id RAA19416; Tue, 24 Dec 1996 17:07:47 -0800 (PST) Received: (from root@localhost) by dyson.iquest.net (8.8.2/8.6.9) id UAA01427; Tue, 24 Dec 1996 20:06:28 -0500 (EST) From: "John S. Dyson" Message-Id: <199612250106.UAA01427@dyson.iquest.net> Subject: Re: DAT: reading with blocksize=256K To: se@freebsd.org (Stefan Esser) Date: Tue, 24 Dec 1996 20:06:23 -0500 (EST) Cc: joerg_wunsch@uriah.heep.sax.de, freebsd-current@freebsd.org, cg@archimedia.khs-linz.ac.at In-Reply-To: from "Stefan Esser" at Dec 25, 96 01:44:01 am X-Mailer: ELM [version 2.4 PL24 ME8] MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-current@freebsd.org X-Loop: FreeBSD.org Precedence: bulk > On Dec 24, j@uriah.heep.sax.de (J Wunsch) wrote: > > As Christian Gusenbauer wrote: > > > > > I just got a DAT cartridge with a tar backup. It seems that the backup > > > was made with a blocksize of 256K. Isn't it possible to get the data into > > > my pc with current (it looks like there's a limit of 64K)?? > > > > This has been discussed at lenth already: it's currently limited by > > physio(9) to chunks of at most 64 KB size, due to the limitations in > > the scatter/gather list of some SCSI controllers that don't allow for > > more than 16 scatter/gather segments. > > I can't talk for any other driver, but in case of the NCR driver, > a simple rebuild of the kernel with MAX_SCATTER (in /sys/pci/ncr.c) > set to 129 will allow to read 256KB tape blocks. > Since it is that simple in the driver level for at least one of our SCSI interfaces, then it might be desirable to start supporting larger physio's. I have a bit of technology in mind to support bigger I/O. Since I use NCR controllers, I'll look into it soon. It would be neat to also support larger than 64K clustered reads/writes. Thanks for the info!!! John dyson@freebsd.org