Date: Thu, 11 Dec 2003 14:21:55 -0600 From: Dan Nelson <dnelson@allantgroup.com> To: Matthew Seaman <m.seaman@infracaninophile.co.uk>, Dru <dlavigne6@sympatico.ca>, questions@freebsd.org Subject: Re: dd of mounted filesystem Message-ID: <20031211202155.GK2435@dan.emsphone.com> In-Reply-To: <20031211201144.GD75256@happy-idiot-talk.infracaninophile.co.uk> References: <20031211145245.D637@genisis> <20031211201144.GD75256@happy-idiot-talk.infracaninophile.co.uk>
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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
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