From owner-freebsd-current@FreeBSD.ORG Thu May 26 18:12:53 2005 Return-Path: X-Original-To: freebsd-current@freebsd.org Delivered-To: freebsd-current@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 26C1F16A41C for ; Thu, 26 May 2005 18:12:53 +0000 (GMT) (envelope-from anderson@centtech.com) Received: from mh1.centtech.com (moat3.centtech.com [207.200.51.50]) by mx1.FreeBSD.org (Postfix) with ESMTP id B129A43D48 for ; Thu, 26 May 2005 18:12:52 +0000 (GMT) (envelope-from anderson@centtech.com) Received: from [10.177.171.220] (neutrino.centtech.com [10.177.171.220]) by mh1.centtech.com (8.13.1/8.13.1) with ESMTP id j4QICprg080732; Thu, 26 May 2005 13:12:51 -0500 (CDT) (envelope-from anderson@centtech.com) Message-ID: <42961195.30608@centtech.com> Date: Thu, 26 May 2005 13:12:37 -0500 From: Eric Anderson User-Agent: Mozilla/5.0 (X11; U; FreeBSD i386; en-US; rv:1.7.7) Gecko/20050504 X-Accept-Language: en-us, en MIME-Version: 1.0 To: Scott Long References: <4295D51F.50106@centtech.com> <429606D9.6080602@cs.tu-berlin.de> <42960ACB.7090801@cs.tu-berlin.de> <42960CFE.4060307@centtech.com> <42960F8F.2050109@samsco.org> In-Reply-To: <42960F8F.2050109@samsco.org> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit X-Virus-Scanned: ClamAV 0.82/894/Wed May 25 07:53:16 2005 on mh1.centtech.com X-Virus-Status: Clean Cc: FreeBSD Current Subject: Re: Disable read/write caching to disk? X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 26 May 2005 18:12:53 -0000 Scott Long wrote: > Eric Anderson wrote: > >> Bjoern Koenig wrote: >> >>> Bjoern Koenig wrote: >>> >>>> Eric Anderson wrote: >>>> >>>>> Is it possible to disable all read and write caching to a disk? >>>> >>>> >>>> >>>> >>>> You can disable write the cache by adding the line >>>> >>>> hw.ata.wc="0" >>> >>> >>> >>> >>> I assumed that you use ATA. If you use SCSI devices then read at >>> least the manpages da(4) and camcontrol(8). >> >> >> >> Thanks.. I've just read (quickly) both man pages. It seems as though >> you are suggesting disabling the physical disk caching, which should >> not make a difference in my case. The disk would report whatever it >> needs to report to either host, and those should be in sync. >> >> When I mount the filesystem on host B ro, it shows me the filesystem >> as of the time that I mounted it ro. Any subsequent changes on host A >> (which has it mounted rw) are not seem on host B unless I unmount and >> mount again on host B. This seems like a FreeBSD feature and not a >> general scsi feature. >> >> Eric >> >> >> >> > > You simply cannot disable OS caching in FreeBSD. It's a fundamental > part of the block I/O and VM layers. There are filesystems like GFS > that deal with the issue of directly connecting more than one computer > to a disk or set of disks, and there are distributed filesystems like > AFS and Coda that deal with making the storage on multiple computers > appear as a single network filesystem. Unfortunately, no port of GFS > has been done yet, and I estimate that such a port would take 4-6 months. Thanks Scott. I know of GFS, and would *love* to see it ported to FreeBSD. I wonder if there is a group of developers that would be willing to do that? I understand what I am doing is 'illegal', but I'm wondering why the ro mount only sees the changes from the time of ro mount. Mounting rw also shows the same thing. Do you think it's a caching issue, or something with UFS that makes it work this way? I'm in no way advocating doing this, nor am I saying 'it should work' - I'm trying to learn more, understand it, and maybe use it as a failover mechanism. Does anyone know the real dangers of forcing an unclean UFS filesystem mounted rw and skipping the fsck? Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology A lost ounce of gold may be found, a lost moment of time never. ------------------------------------------------------------------------