• Stefan Behrens's avatar
    Btrfs: introduce a tree for items that map UUIDs to something · 07b30a49
    Stefan Behrens authored
    
    
    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.
    
    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 with this commit that is able to
    maintain data structures in the filesystem that allow to quickly
    search for a given UUID and to retrieve data that is assigned to
    this UUID, like which subvolume ID is related to this UUID.
    
    This commit adds a new tree to hold UUID-to-data mapping items. The
    key of the items is the full UUID plus the key type BTRFS_UUID_KEY.
    Multiple data blocks can be stored for a given UUID, a type/length/
    value scheme is used.
    
    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.
    
    Signed-off-by: default avatarStefan Behrens <sbehrens@giantdisaster.de>
    Signed-off-by: default avatarJosef Bacik <jbacik@fusionio.com>
    Signed-off-by: default avatarChris Mason <chris.mason@fusionio.com>
    07b30a49