From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.fusionio.com ([66.114.96.30]:34082 "EHLO mx1.fusionio.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1030243Ab2GFLz1 (ORCPT ); Fri, 6 Jul 2012 07:55:27 -0400 Date: Fri, 6 Jul 2012 07:55:24 -0400 From: Chris Mason To: Alexander Block CC: "kreijack@inwind.it" , "linux-btrfs@vger.kernel.org" Subject: Re: [RFC] Btrfs "sendshots" and hidden snapshots Message-ID: <20120706115524.GH7159@shiny> References: <4FF61669.9080605@libero.it> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" In-Reply-To: Sender: linux-btrfs-owner@vger.kernel.org List-ID: On Fri, Jul 06, 2012 at 02:51:43AM -0600, Alexander Block wrote: > On Fri, Jul 6, 2012 at 12:34 AM, Goffredo Baroncelli wrote: > > On 07/05/2012 06:51 PM, Alexander Block wrote: > >> Hello all, > >> > >> in IRC we had a discussion on how we could solve sending live > >> subvolumes and how to send subvolumes without the need to > >> administrate/keep old snapshots for incremental sends. One of the > >> ideas was to introduce "sendshots", which are basically snapshots > >> where no refs are counted for file data. This means, that when file > >> data is changed in the sendshot origin, we do not consume extra space > >> for two copies of the data. We would only have the metadata > >> duplicated. > >> > >> For the initial btrfs send we could do this: > >> 1. Create a hidden read-only snapshot of the subvolume to send. Hidden > >> means that it's not referenced by any subvolume. It is however still a > >> normal snapshot (not a sendshot!). Hidden snapshots are not possible > >> atm so we would have to implement that. This step allows us to send > >> read-write subvolumes, because we have a freezed version of it. > > > > Why we should want/need an hidden snapshot ? We could put this kind of > > hidden snapshot under a directory dot-prefixed (like /.hidden-subvolumes) > That would have the problem that the user may modify the subvolume > in-between (by removing the ro flag). Or he could simple cd into it > and we would later fail to delete it. I prefer to make this more explicit. We could add a hard-readonly flag that cannot be cleared. Having the snapshot show in the FS lets the admin know what things are really using space. -chris