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. > 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. > Probably before that, I need to get some clarity on why does a > subvolume always belong in the directory of some parent subvolume. Is > it possible to have more than one root subvolumes or more than one > subvolumes in the same parent subvolume directory? No, there's precisely one top-level subvolume (subvolid=5). Everything else in the filesystem lives within that. However, you can have as many subvolumes as you like below that, and in whatever directories or subvolumes you want. 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 --- Quidquid latine dictum sit, altum videtur. ---
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