On Sun, Jun 27, 2010 at 07:44:12PM -0500, C Anthony Risinger wrote: > 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.
I'm not sure I understand use case #1. The problem is that you'll have files open in the subvolume and you can't just pull the rug out from under them. Could you tell me a little more about what you're trying to do? > > > > 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. Moving 'default' generally involves a reboot for the same reasons. We have to worry about open files and their view of the filesystem. mv on a directory won't affect file handles that are open, and renaming subvolumes needs to follow a similar model. -chris -- 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
