From owner-freebsd-questions@FreeBSD.ORG Thu Dec 11 12:21:57 2003 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 1C25216A4CE for ; Thu, 11 Dec 2003 12:21:57 -0800 (PST) Received: from dan.emsphone.com (dan.emsphone.com [199.67.51.101]) by mx1.FreeBSD.org (Postfix) with ESMTP id 1496843D09 for ; Thu, 11 Dec 2003 12:21:56 -0800 (PST) (envelope-from dan@dan.emsphone.com) Received: (from dan@localhost) by dan.emsphone.com (8.12.10/8.12.10) id hBBKLtxP078022; Thu, 11 Dec 2003 14:21:55 -0600 (CST) (envelope-from dan) Date: Thu, 11 Dec 2003 14:21:55 -0600 From: Dan Nelson To: Matthew Seaman , Dru , questions@freebsd.org Message-ID: <20031211202155.GK2435@dan.emsphone.com> References: <20031211145245.D637@genisis> <20031211201144.GD75256@happy-idiot-talk.infracaninophile.co.uk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20031211201144.GD75256@happy-idiot-talk.infracaninophile.co.uk> X-OS: FreeBSD 5.2-BETA X-message-flag: Outlook Error User-Agent: Mutt/1.5.5.1i Subject: Re: dd of mounted filesystem X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 11 Dec 2003 20:21:57 -0000 In the last episode (Dec 11), Matthew Seaman said: > On Thu, Dec 11, 2003 at 02:54:12PM -0500, Dru wrote: > > Can anyone describe or point me to resources explaining why it is > > dangerous to dd a filesystem while it is mounted? Is it still > > considered to be dangerous if the system is first dropped down to > > single-user mode? > > Remember that dd(1) traverses the block device sequentially, but that > most FS accesses are random, so any particular change can span either > side of dd(1)'s offset. Also that dd'ing from the block device > bypasses the usual machinery for doing file IO -- machinery that is > designed under the premise that it will have sole control over what > gets read or written where and when. On current you can get around the consistency problem by dd'ing a snapshot of the filesystem, just like dump's -L flag does. -- Dan Nelson dnelson@allantgroup.com