From owner-freebsd-fs@FreeBSD.ORG Wed May 3 14:04:30 2006 Return-Path: X-Original-To: freebsd-fs@freebsd.org Delivered-To: freebsd-fs@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 69ABD16A406 for ; Wed, 3 May 2006 14:04:30 +0000 (UTC) (envelope-from rwatson@FreeBSD.org) Received: from cyrus.watson.org (cyrus.watson.org [209.31.154.42]) by mx1.FreeBSD.org (Postfix) with ESMTP id C06C243D49 for ; Wed, 3 May 2006 14:04:29 +0000 (GMT) (envelope-from rwatson@FreeBSD.org) Received: from fledge.watson.org (fledge.watson.org [209.31.154.41]) by cyrus.watson.org (Postfix) with ESMTP id 0A90A46C33; Wed, 3 May 2006 10:04:23 -0400 (EDT) Date: Wed, 3 May 2006 15:04:22 +0100 (BST) From: Robert Watson X-X-Sender: robert@fledge.watson.org To: Pavel Merdin In-Reply-To: <1408111762.20060503145710@merdin.com> Message-ID: <20060503150126.K21389@fledge.watson.org> References: <20060502193900.GA94069@peter.osted.lan> <1541458526.20060503003229@merdin.com> <20060502221306.GD95348@xor.obsecurity.org> <44584421.3000807@cs.tu-berlin.de> <20060503072013.GA2926@xor.obsecurity.org> <18034.193.3.141.124.1146642890.squirrel@webmail7.pair.com> <1408111762.20060503145710@merdin.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Cc: freebsd-fs@freebsd.org Subject: Re[2]: Stress testing the UFS2 filesystem X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 03 May 2006 14:04:31 -0000 On Wed, 3 May 2006, Pavel Merdin wrote: >> Actually the filesystems mounts without any problems if fsck is run first > > That's not a bug in this case. It's a feature. And there is nothing new in > this. Background fsck helps saving start time, but it's risky as kernel can > panic if system accesses problematic sector before fsck. So background fsck > should be turned off if one needs reliability (e.g. on servers). Well, the feature and problem are that bgfsck relies on invariants holding true for data written to the disk, so corrects only a narrow set of expected failure modes. I.e., that soft updates really does sequence changes out to the disk such that certain invariants regarding meta-data hold. This allows access to the file system before the checks are complete, but places assumptions on what will be found on disk. Those invariants might fail to hold for a few reasons -- bugs in UFS, hardware failure, and design breakage in the hardware are among the most common. One known problem is that the guarantees provided by recent ATA disks are really very weak with respect to the expectations of currently file systems. Robert N M Watson