From mboxrd@z Thu Jan 1 00:00:00 1970 From: Zheng Yan Subject: Re: snapshot merging? Date: Sun, 21 Sep 2008 23:00:14 +0800 Message-ID: <48D6617E.1010908@oracle.com> References: <48D63C8E.2060802@wpkg.org> <48D657DB.7050601@oracle.com> <48D65ACD.7030907@wpkg.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Cc: linux-btrfs@vger.kernel.org To: Tomasz Chmielewski Return-path: In-Reply-To: <48D65ACD.7030907@wpkg.org> List-ID: Tomasz Chmielewski wrote: > Zheng Yan schrieb: >> Tomasz Chmielewski wrote: >>> At the beginning of August, Mikulas Patocka posted to linux-kernel >>> mailing list about adding snapshot merging to LVM[1]. >>> >>> Basicaly, snapshot merging means that it is possible to turn a snapshot >>> back into its origin. >>> >>> Using LVM, however, means that you need to have free place outside of >>> the filesystem (i.e., in physical volume) to make snapshots, which is >>> not always possible on workstations and some servers. >>> >>> Is something similar available, or planned, for btrfs? I didn't find >>> anything similar on "btrfs design" page in the wiki. >>> >> Writeable snapshot has been available in btrfs for a long time. > > Writeable snapshots and snapshot merging are two different things. > > >>> [1] http://kerneltrap.org/Linux/LVM_Snapshot_Merging >>> >> Writeable snapshot means you can mount/modify snapshots as you like. >> In other words, btrfs does snapshot merging automatically. > > With LVM, you can also modify snapshots as you like, but it doesn't mean > LVM supports snapshot merging right now (that is, you have to apply the > patches described on the page above to have "snapshot merging" feature). > > > Maybe let me show you an example: > > 0) you have your "original" root filesystem > 1) you make a snapshot of your rootfs > 2) you upgrade your distribution (version 1.0 -> version 2.0-alpha) > 3) you realise that some of the things don't work as expected - you mark > that your snapshot should be turned into an original > 4) you reboot (as your rootfs is mounted, you have to) and from now on > your snapshot turns into the original from point 0. > You can mount the snapshot created in step 1 as your rootfs. (by changing rootfs mount option in the boot parameter) Yan Zheng