From owner-freebsd-current Sat Jan 16 02:46:45 1999 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id CAA29726 for freebsd-current-outgoing; Sat, 16 Jan 1999 02:46:45 -0800 (PST) (envelope-from owner-freebsd-current@FreeBSD.ORG) Received: from vader.cs.berkeley.edu (vader.CS.Berkeley.EDU [128.32.38.234]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id CAA29721 for ; Sat, 16 Jan 1999 02:46:44 -0800 (PST) (envelope-from asami@vader.cs.berkeley.edu) Received: from silvia.hip.berkeley.edu (sji-ca36-130.ix.netcom.com [207.92.172.130]) by vader.cs.berkeley.edu (8.8.7/8.7.3) with ESMTP id CAA09567; Sat, 16 Jan 1999 02:46:34 -0800 (PST) Received: (from asami@localhost) by silvia.hip.berkeley.edu (8.9.1/8.6.9) id CAA47630; Sat, 16 Jan 1999 02:46:30 -0800 (PST) Date: Sat, 16 Jan 1999 02:46:30 -0800 (PST) Message-Id: <199901161046.CAA47630@silvia.hip.berkeley.edu> To: phk@critter.freebsd.dk CC: current@FreeBSD.ORG In-reply-to: <9602.916482530@critter.freebsd.dk> (message from Poul-Henning Kamp on Sat, 16 Jan 1999 11:28:50 +0100) Subject: Re: mounting double-ended SCSI disks From: asami@FreeBSD.ORG (Satoshi Asami) Sender: owner-freebsd-current@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG * >However, if I try to mount it from B read-only while A is mounting it * >read-write, it succeeds. This looks dangerous, as A writing data onto * >the disk could cause B's cache to go stale without B knowing it. Is * >it a good idea to allow read-only mounts of a dirty filesystem anyway? * >(The filesystem could be corrupted, right?) * * UFS/FFS doesn't expect anybody else to muck about on the device * while they have it open, and violating that is a bad idea, I cannot I know that, but that's not the point here. If the filesystem is marked dirty, it could very well be corrupted. Why am I allowed to mount it (even read-only)? I use softupdates on these filesystems, and my understanding is that it is theoretically safe to mount a filesystem without fsck after a crash if it's using softupdates. But I don't see mount checking that (if it is, it should allow read-write mounts too -- mount -f is not quite the same thing is it will override the check even for non-softupdates case). * tell if it would lead to panics, but I can imagine a couple of ways * it would become quantum mechanical in such a setup. * A couple of filesystem have been designed over the years which allow * for multiple machine access, but they tend to have lousy performance * because of caching being so inefficient. One of the better * implementations cheated, they stored the stuff in an Oracle database * on a third machine, but used a filesystem interface... To clarify, we're not trying to build a distributed filesystem here. We're planning to use the disks from both machines read-only most of the time, and unmount it from one machine if the other needs to write to it. I was just wondering what kind of safety belts the OS already has, so we can decide what else we need to implement. Satoshi To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message