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. Regards, Daniel -- Address: Daniel Baumann, Burgunderstrasse 3, CH-4562 Biberist Email: daniel.baum...@panthera-systems.net Internet: http://people.panthera-systems.net/~daniel-baumann/ -- 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
