From owner-freebsd-hackers Tue Nov 10 22:48:07 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id WAA08628 for freebsd-hackers-outgoing; Tue, 10 Nov 1998 22:48:07 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from allegro.lemis.com (allegro.lemis.com [192.109.197.134]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id WAA08605 for ; Tue, 10 Nov 1998 22:48:04 -0800 (PST) (envelope-from grog@freebie.lemis.com) Received: from freebie.lemis.com (freebie.lemis.com [192.109.197.137]) by allegro.lemis.com (8.9.1/8.9.0) with ESMTP id PAA21059; Wed, 11 Nov 1998 15:16:30 +1030 (CST) Received: (from grog@localhost) by freebie.lemis.com (8.9.1/8.9.0) id PAA20597; Wed, 11 Nov 1998 15:16:28 +1030 (CST) Message-ID: <19981111151628.J20374@freebie.lemis.com> Date: Wed, 11 Nov 1998 15:16:28 +1030 From: Greg Lehey To: Julian Elischer , Peter Jeremy Cc: hackers@FreeBSD.ORG Subject: Re: SCSI tagged queueing and softupdates References: <98Nov11.084659est.40344@border.alcanet.com.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.91.1i In-Reply-To: ; from Julian Elischer on Tue, Nov 10, 1998 at 08:16:08PM -0800 WWW-Home-Page: http://www.lemis.com/~grog Organization: LEMIS, PO Box 460, Echunga SA 5153, Australia Phone: +61-8-8388-8286 Fax: +61-8-8388-8725 Mobile: +61-41-739-7062 Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On Tuesday, 10 November 1998 at 20:16:08 -0800, Julian Elischer wrote: > 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. > > transactions can be reorderd AS LONG AS the drive does not report a > perticular transaction to be completed before it really is.. As long as you define "transaction" correctly. SCSI-level transactions must often be completed in order. I rely on this (and the associated B_ORDERED flag in the buffer header) to ensure consistency in Vinum. > 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) Doesn't this slow the whole thing down? Or is that what's behind B_ORDERED? Greg -- See complete headers for address, home page and phone numbers finger grog@lemis.com for PGP public key To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message