From owner-freebsd-hackers Tue Nov 10 20:27:23 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id UAA03943 for freebsd-hackers-outgoing; Tue, 10 Nov 1998 20:27:23 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from alpo.whistle.com (alpo.whistle.com [207.76.204.38]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id UAA03938 for ; Tue, 10 Nov 1998 20:27:21 -0800 (PST) (envelope-from julian@whistle.com) Received: (from daemon@localhost) by alpo.whistle.com (8.8.5/8.8.5) id UAA25990; Tue, 10 Nov 1998 20:16:48 -0800 (PST) Received: from current1.whistle.com(207.76.205.22) via SMTP by alpo.whistle.com, id smtpds25988; Wed Nov 11 04:16:40 1998 Date: Tue, 10 Nov 1998 20:16:08 -0800 (PST) From: Julian Elischer To: Peter Jeremy cc: hackers@FreeBSD.ORG Subject: Re: SCSI tagged queueing and softupdates In-Reply-To: <98Nov11.084659est.40344@border.alcanet.com.au> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG transactions can be reorderd AS LONG AS the drive does not report a perticular transaction to be completed before it really is.. Soft updates will not schedule any IO that depends on an uncompleted IO operation. So therefore all operations that have been 'passed to the driver' are not dependent on each other (by definition) On Wed, 11 Nov 1998, Peter Jeremy wrote: > AFAIK, SCSI tagged queueing is the default in 3.0 (via cam or inside > the ahc driver). (This was optionally enabled via AHC_TAGENABLE in 2.x > and the 3.0 man page still describes this option, although the option > is gone). > > According to the description, tagged queueing allows a SCSI device to > re-order pending I/O requests in order to improve throughput. Soft > updates, on the other hand, rely on the correct sequencing of physical > writes to ensure that the disk metadata is consistent. > > Is this a real issue? If so, it would seem that tagged queueing must > be disabled (at least for writes) when softupdates are enabled. It's > not obvious to me that this is currently done (or even easy to do). > > Comments please. > > Peter > -- > Peter Jeremy (VK2PJ) peter.jeremy@alcatel.com.au > Alcatel Australia Limited > 41 Mandible St Phone: +61 2 9690 5019 > ALEXANDRIA NSW 2015 Fax: +61 2 9690 5247 > > To Unsubscribe: send mail to majordomo@FreeBSD.org > with "unsubscribe freebsd-hackers" in the body of the message > To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message