From mboxrd@z Thu Jan 1 00:00:00 1970 From: Yan Zheng Subject: Re: Seed device questions Date: Fri, 28 Nov 2008 21:19:01 +0800 Message-ID: <492FEFC5.2050009@gmail.com> References: <492FC847.1090605@southpole.se> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: linux-btrfs@vger.kernel.org To: Jonas Bonn Return-path: In-Reply-To: <492FC847.1090605@southpole.se> List-ID: Jonas Bonn wrote: > Hi, > I am trying the 'seed device' functionality, trying to get a read-only > block-device to act as a seed for multiple other filesystems. > > The following sequence works for setting up the first filesystem. > > #------------- > > #First initialize 'seed device' > mkfs.btrfs /dev/vdb > mount /dev/vdb /mnt > btrfs -S subvol /mnt > echo hello > /mnt/subvol/xxx > umount /mnt > > #Make block device a "seed device" (read-only) > btrfstune -S 1 /dev/vdb > > #Mount the seed device... it will be mounted read-only. > mount /dev/vdb /mnt > > #Add the writeable block device > btrfs-vol -a /dev/vdc /mnt > > #Now remount the filesystem read-write. > mount -oremount,rw /mnt > > #Now all changes to /mnt should go to /dev/vdc. > > #-------------- > > After this, however, it's not possible to mount /dev/vdb again in order > to set up a second filesystem as seed. I want to do: > > mount /dev/vdb /othermnt > btrfs-vol -a /dev/vdd /othermnt > mount -oremount,rw /othermnt > > So my questions (comments): > > i) Is the seed-device _really_ read-only... no metadata updates or > anything? (i.e. I can have my seed device on read-only media) yes > ii) Should it be possible to have a device as a 'seed' for multiple > other filesystems. it's possible, but only one filesystem can be mounted at a time. I will add 'mounting multiple filesystems at the same time' support later. > iii) When the seeded filesystem is created, the size of the filesystem > is the aggregate of all devices, whereas it should be the aggregate size > of only the 'writable' devices. > Since btrfs mirrors metadata and creates chunk dynamically, how to compute the size of filesystem isn't clear. I'm afraid I can't answer this question. Regards Yan Zheng