Thanks a lot for the explanation. Regards, Aastha.
On 3 December 2012 13:02, Hugo Mills <h...@carfax.org.uk> wrote: > On Mon, Dec 03, 2012 at 10:52:41AM +0100, Aastha Mehta wrote: >> On 2 December 2012 23:46, Hugo Mills <h...@carfax.org.uk> wrote: >> > On Sun, Dec 02, 2012 at 11:17:26PM +0100, Aastha Mehta wrote: >> >> I am looking at btrfs to understand some of its features. One of them >> >> is the snapshot feature. Please tell me if my following understanding >> >> about snapshots in btrfs is correct or not. >> >> >> >> Btrfs supports both readonly and writeable snapshots. Writeable >> >> snapshots are like clone volumes (or subvolumes as in btrfs). We get a >> >> point in time copy of the subvolume in both case. >> >> >> >> I looked through the kernel code and it seems that creating a >> >> subvolume and taking a snapshot (readonly and writeable) all have a >> >> common ioctl interface. >> >> >> >> What I am not completely clear about is whether snapshots get same >> >> fsid as the source subvolume fsid or different. >> > >> > Yes, it's the same UUID, because they're all part of the same >> > filesystem. >> > >> Just to clarify, apart from UUID, is the FSID in the fs_info of the >> root also same for all snapshots of a subvolume? >> >> >> Also, I do not understand what does it mean to be able to take >> >> snapshot of a snapshot. >> > >> > Snapshots are completely equal partners with their original >> > subvolumes. This is not the case in, say, LVM. >> > >> >> What are benefits compared to say, being able to take snapshots only >> >> of the active subvolume and not of the snapshots? >> > >> > Let's say you take a snapshot (B) of your root filesystem (A). Then >> > you decide to "roll back" to using the old version, so you mount B as >> > root instead of A. Later that night, your backup process starts up and >> > tries to take a temporary read-only snapshot (C) of your root >> > filesystem (which is now B) so that it can make a stable backup. >> > That's a snapshot of a snapshot. >> > >> Okay, but still the snapshot can be taken only for a subvolume in use. >> Is that correct? > > Well, it depends on what you mean by "in use". You can't snapshot > something which doesn't appear somewhere in your directory hierarchy. > >> In your example, C is taken on B after file system >> was rolled back to version B. What happens when the file system >> version mounted is A (which contains snapshot B) and we take another >> snapshot D on this mounted version. Does the snapshot D contain B or >> only the active contents of A? > > Snapshots are not recursive. If you have a subvolume inside another: > > subv1/subv2 > > and then snapshot that > > # btrfs sub snap subv1 subv1-A > > you will end up with a subvolume subv1-A, containing an empty > directory called subv2. > > Note that you don't have to have subvolumes inside subvolumes at > all, if you don't use the top level of your filesystem as anything > other than a place to store and manage subvolumes. > > Consider this btrfs filesystem layout: > > / subvolid=5 (=0) > root subvolid=256 (default subvol) > home subvolid=257 > snapshots directory > > With this in fstab: > > /dev/sda / btrfs subvolid=256 > /dev/sda /home btrfs subvolid=257 > /dev/sda /media/btrfs btrfs subvolid=5,noauto > > We get this filesystem hierarchy: > > / subvolid=256 > home subvolid=257 > media directory > btrfs subvolid=5 > > Note that the mount of the full filesystem on /media/btrfs isn't > done automatically -- it only needs to be done when you're managing > subvolumes. > > So, we can take a snapshot of /, for example: > > # mount /media/btrfs > # btrfs sub snap /media/btrfs/root /media/btrfs/snapshots/root.2012-12-03 > # umount /media/btrfs > > The FS (from its top level) now looks like this: > > / subvolid=5 (=0) > root subvolid=256 (default subvol) > home subvolid=257 > snapshots directory > root.2012-12-03 subvolid=258 > > To "roll back" root temporarily to the earlier version, you can edit > your boot manager's config to supply "subvolid=258" as a mount > parameter. To do so permanently, you can set the default subvolume to > 258, and optionally move the snapshot to /root within the btrfs > filesystem: > > # mount /media/btrfs > # mv /media/btrfs/root /media/btrfs/root.old > # mv /media/btrfs/snapshots/root.2012-12-03 /media/btrfs/root > # umount /media/btrfs > > Hugo. > > -- > === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === > PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk > --- I'm on a 30-day diet. So far I've lost 18 days. --- -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
