Package: btrfs-tools Version: 0.19+20100601-3 Severity: normal 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 I've filed this against btrfs-tools, but it may well need support in-kernel as well as support in the btrfs tool. Please could you forward this to the appropriate upstream(s)? Regards, Roger -- System Information: Debian Release: squeeze/sid APT prefers unstable APT policy: (550, 'unstable') Architecture: amd64 (x86_64) Kernel: Linux 2.6.32-5-amd64 (SMP w/4 CPU cores) Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8) Shell: /bin/sh linked to /bin/dash Versions of packages btrfs-tools depends on: ii e2fslibs 1.41.12-2 ext2/ext3/ext4 file system librari ii libc6 2.11.2-2 Embedded GNU C Library: Shared lib ii libcomerr2 1.41.12-2 common error description library ii libuuid1 2.17.2-3.1 Universally Unique ID library ii zlib1g 1:1.2.3.4.dfsg-3 compression library - runtime btrfs-tools recommends no packages. btrfs-tools suggests no packages. -- no debconf information -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
