On Wed, Jun 30, 2010 at 09:26:11AM -0500, C Anthony Risinger wrote: > On Wed, Jun 30, 2010 at 8:31 AM, Chris Mason <chris.ma...@oracle.com> wrote: > > 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. > > could we fail if the user tries to replace a subvolume while it's > being used? what if the root device is _not_ the default (".") > subvolume, then can it be swapped? > > in my use case, i am running in initramfs, so the root device has not > even been mounted or pivoted to; it should be safe to do whatever i > want to the filesystem. i want to move the user's installation to a > dedicated subvolume. > > what about this: would it be possible to have TWO subvolumes by > "default"? the regular one (current directory, "."): > > mount -o subvol=. <btrfs_dev> /mnt > > would behave as it does now. BUT... there would then be a special, > permanent (like "." is right now) subvol, say "parent directory" > (".."): > > mount -o subvol=.. <btrfs_dev> /mnt > > TWO dots would mount the parent of ".", where i could then swap out > the real default ("."). > > would that work?
We do provide a set-default ioctl that can be used to change the default for the next mount. This is pretty close to what you want, let me think about ways to make it easier to use. -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
