Hello, This is the second attempt to bring in cross subvolume reflinks into btrfs. The first attempt was NAKed due to missing vfs mount checks and a clear description of what btrfs subvolumes are and probably also why cross subvolume reflinks are ok in the case of btrfs. This version of the patch comes from David and is in SUSE kernels since a long time, so it is tested and working. The patch also does proper vfs mount checks, so cross mount point reflinks are not possible with this patch. It only allows cross reflinks between two subvolumes which are in the same mount point.
As requested in the older discussion, I'll try to describe what subvolumes in btrfs are. From the users point of view, a subvolume is just a directory in the mounted fs with its own inode space. This means, that two inodes with the same number from different subvolumes are different files. Parallel to the sub directory form, a subvolume may be mounted in its own vfs mount point (I'm however not sure if this is already possible or just planned). Internally, a btrfs subvolume is a separate btree, containing all the metadata needed to store the fs of the subvolume. Additionally to the subvolumes, btrfs has snapshots, which are basically the same as subvolumes, with the difference that they share metadata and data with the snapshot origin/parent. Everything that applies to subvolumes also applies to snapshots. The clone ioctl requires the destination file to be already existing and opened for writing. This means that we do not touch any vfs semantics regarding creation/opening of files. The ioctl only copies the extents information from the source to the destination, doing proper access checks before. In my opinion, we can compare the clone ioctl to the sendfile syscall, with the difference that we don't copy data but the information where the data lies. Link to the old discussion: http://thread.gmane.org/gmane.comp.file-systems.btrfs/9864 Only PATCH 2/2 is of interrest now. The discussion also contains some arguments why cross subvolume reflinks are a good thing, but I think this should not be the matter of the new discussion. Alex. David Sterba (1): btrfs: allow cross-subvolume file clone fs/btrfs/ioctl.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) -- 1.7.10 -- 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
