Hello, Thank you so much for your prompt response. Few more questions inline.
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? 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? >> 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. --- Thanks again, Aastha. -- 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
