On Sat, Jun 26, 2010 at 12:25 PM, Daniel Baumann <dan...@debian.org> wrote: > Hi, > > this is basically a forward from > http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=587253 > > "rename(2) allows for the atomic replacement of files. Being able to > atomically replace subvolume snapshots would be equally invaluable, > since it would permit lock-free replacement of subvolumes. > > % btrfs subvolume snapshot <src> <dest> > > creates dest as a snapshot of src. However, if I want to do the > converse, > > % btrfs subvolume snapshot <dest> <src> > > then <dest> is snapshotted as <src>/<dest>, i.e. not replacing the > original subvolume, but going inside the original subvolume. > > Use case 1: > I have a subvolume of data under active use, which I want to > periodically update. I'd like to do this by atomically > replacing its contents. I can replace the content right now > by deleting the old subvolume and then snapshotting the new > on in its place, but it's racy. It really needs to be > replaced in a single operation, or else there's a small window > where there is no data, and I'd need to resort to some external > locking to protect myself. > > Use case 2: > In schroot, we create btrfs subvolume snapshots to get copy-on- > write chroots. This works just fine. We also provide direct > access to the "source" subvolume, but since it could be > snapshotted in an inconsistent state while being updated, we > want to do the following: > > · snapshot source subvolume > · update snapshot > · replace source volume with updated snapshot" > > Please keep roger in the cc for any replies, thanks.
i am also looking for functionality similar to this, except i would like to be able to replace the DEFAULT subvolume, with an empty or existing subvolume, and put the original default subvolume INSIDE the new root (or drop it completely), outlined by this post and the thread it's in: http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg05278.html is there any feedback on these actions? no one seems to even respond :-( it would seem we need ways to swap subvolumes around, _including_ the default, providing the on-disk format supports such operations. C Anthony -- 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
