From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Wolfgang Weisselberg Date: Thu, 26 Jul 2001 13:55:14 +0200 Subject: Re: [linux-lvm] Some ideas regarding snapshots (was: Reiserfs, 2.4.7, snapshots w/ 0.9) Message-ID: <20010726135514.B26425@tiger.bigcats.invalid> References: <20010725103640.J5363@unirez.com> <01072511465701.00921@exodus> Mime-Version: 1.0 Content-Disposition: inline In-Reply-To: <01072511465701.00921@exodus>; from iain@sandon.ca on Wed, Jul 25, 2001 at 11:46:57AM -0400 Sender: linux-lvm-admin@sistina.com Errors-To: linux-lvm-admin@sistina.com Reply-To: linux-lvm@sistina.com List-Help: List-Post: List-Subscribe: , List-Unsubscribe: , List-Archive: List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-lvm@sistina.com Iain Campbell (iain@sandon.ca) wrote 64 lines: > Quote from Teigland and Mauelshagens's Volume Manager whitepaper: > journalled file system. These operations (fsck or journal replay) require > writing to the snapshot ligical volume, which is not allowed." On a related subject: would it be possible to have a writable snapshot (probably a 'reverse' snapshot)? I remember a few situations where I would have done a lot for a feature like that; e.g. building a system that's 'guaranteed' the same after restarting/rebooting it. The idea is along the lines of: - you have the system in a known good, fscked state on the HD. - you snapshot that (possibly huge) system - you mount the snapshots r/w - the original will probably stay unmounted or r/o (but not neccessarily, I am thinking of robust, networked stand-alone devices which could be upgraded in the background while running; a simple restart will have the upgrade applied.) - The system can scribble onto the snapshots until they are full. - To reset the complete system to the known good state, umount, unsnapshot, start over. Or even -- horror -- reboot. This should be *much* faster than a reintarnation or the use of dd, not to mention easier, simpler, more elegant and using less space. And on another related idea: can we have the option of making snapshots non-persistant again? Or is that lots of work? There sometimes are cases where snapshots lose all value over a reboot, so they could/should automagically disappear. -Wolfgang