From owner-freebsd-questions@FreeBSD.ORG Fri Dec 12 05:41:45 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 849F116A4CE for ; Fri, 12 Dec 2003 05:41:45 -0800 (PST) Received: from tomts22-srv.bellnexxia.net (tomts22.bellnexxia.net [209.226.175.184]) by mx1.FreeBSD.org (Postfix) with ESMTP id 0299143D1F for ; Fri, 12 Dec 2003 05:41:44 -0800 (PST) (envelope-from dlavigne6@sympatico.ca) Received: from genisis ([64.230.81.232]) by tomts22-srv.bellnexxia.net (InterMail vM.5.01.06.05 201-253-122-130-105-20030824) with ESMTP id <20031212134143.CWAX26187.tomts22-srv.bellnexxia.net@genisis>; Fri, 12 Dec 2003 08:41:43 -0500 Date: Fri, 12 Dec 2003 08:43:21 -0500 (EST) From: Dru X-X-Sender: dlavigne6@genisis.domain.org To: Dan Nelson In-Reply-To: <20031211202155.GK2435@dan.emsphone.com> Message-ID: <20031212084227.Q604@genisis.domain.org> References: <20031211145245.D637@genisis> <20031211201144.GD75256@happy-idiot-talk.infracaninophile.co.uk> <20031211202155.GK2435@dan.emsphone.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII cc: questions@freebsd.org 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: Fri, 12 Dec 2003 13:41:45 -0000 On Thu, 11 Dec 2003, Dan Nelson wrote: > 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. You mean, run "makesnap_ffs" first? I've been meaning to play with that one, I'll have to try it out. Dru