Mapping UUIDs to subvolume IDs is an operation with a high effort today. Today, the algorithm even has quadratic effort (based on the number of existing subvolumes), which means, that it takes minutes to send/receive a single subvolume if 10,000 subvolumes exist. But even linear effort would be too much since it is a waste. And these data structures to allow mapping UUIDs to subvolume IDs are created every time a btrfs send/receive instance is started.
So the issue to address is that Btrfs send / receive does not work as it is today when a high number of subvolumes exist. It is much more efficient to maintain a searchable persistent data structure in the filesystem, one that is updated whenever a subvolume/snapshot is created and deleted, and when the received subvolume UUID is set by the btrfs-receive tool. Therefore kernel code is added that is able to maintain data structures in the filesystem that allow to quickly search for a given UUID and to retrieve the subvol ID. Now follows the lengthy justification, why a new tree was added instead of using the existing root tree: The first approach was to not create another tree that holds UUID items. Instead, the items should just go into the top root tree. Unfortunately this confused the algorithm to assign the objectid of subvolumes and snapshots. The reason is that btrfs_find_free_objectid() calls btrfs_find_highest_objectid() for the first created subvol or snapshot after mounting a filesystem, and this function simply searches for the largest used objectid in the root tree keys to pick the next objectid to assign. Of course, the UUID keys have always been the ones with the highest offset value, and the next assigned subvol ID was wastefully huge. To use any other existing tree did not look proper. To apply a workaround such as setting the objectid to zero in the UUID item key and to implement collision handling would either add limitations (in case of a btrfs_extend_item() approach to handle the collisions) or a lot of complexity and source code (in case a key would be looked up that is free of collisions). Adding new code that introduces limitations is not good, and adding code that is complex and lengthy for no good reason is also not good. That's the justification why a completely new tree was introduced. v1 -> v2: - All review comments from David Sterba, Josef Bacik and Jan Schmidt are addressed. The hugest change was to add a mechanism that handles the case that the filesystem is mounted with an older kernel. Now that case is detected when the filesystem is mounted with a newer kernel again, and the UUID tree is updated in the background. Stefan Behrens (8): Btrfs: introduce a tree for items that map UUIDs to something Btrfs: support printing UUID tree elements Btrfs: create UUID tree if required Btrfs: maintain subvolume items in the UUID tree Btrfs: fill UUID tree initially Btrfs: introduce uuid-tree-gen field Btrfs: check UUID tree during mount if required Btrfs: add mount option to force UUID tree checking fs/btrfs/Makefile | 3 +- fs/btrfs/ctree.h | 65 ++++- fs/btrfs/disk-io.c | 62 ++++- fs/btrfs/ioctl.c | 74 +++++- fs/btrfs/print-tree.c | 82 +++++++ fs/btrfs/super.c | 8 +- fs/btrfs/transaction.c | 21 +- fs/btrfs/uuid-tree.c | 637 +++++++++++++++++++++++++++++++++++++++++++++++++ fs/btrfs/volumes.c | 258 ++++++++++++++++++++ fs/btrfs/volumes.h | 2 + 10 files changed, 1197 insertions(+), 15 deletions(-) create mode 100644 fs/btrfs/uuid-tree.c -- 1.8.2.2 -- 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