• Yan Zheng's avatar
    Btrfs: Mixed back reference (FORWARD ROLLING FORMAT CHANGE) · 5d4f98a2
    Yan Zheng authored
    
    
    This commit introduces a new kind of back reference for btrfs metadata.
    Once a filesystem has been mounted with this commit, IT WILL NO LONGER
    BE MOUNTABLE BY OLDER KERNELS.
    
    When a tree block in subvolume tree is cow'd, the reference counts of all
    extents it points to are increased by one.  At transaction commit time,
    the old root of the subvolume is recorded in a "dead root" data structure,
    and the btree it points to is later walked, dropping reference counts
    and freeing any blocks where the reference count goes to 0.
    
    The increments done during cow and decrements done after commit cancel out,
    and the walk is a very expensive way to go about freeing the blocks that
    are no longer referenced by the new btree root.  This commit reduces the
    transaction overhead by avoiding the need for dead root records.
    
    When a non-shared tree block is cow'd, we free the old block at once, and the
    new block inherits old block's references. When a tree block with reference
    count > 1 is cow'd, we increase the reference counts of all extents
    the new block points to by one, and decrease the old block's reference count by
    one.
    
    This dead tree avoidance code removes the need to modify the reference
    counts of lower level extents when a non-shared tree block is cow'd.
    But we still need to update back ref for all pointers in the block.
    This is because the location of the block is recorded in the back ref
    item.
    
    We can solve this by introducing a new type of back ref. The new
    back ref provides information about pointer's key, level and in which
    tree the pointer lives. This information allow us to find the pointer
    by searching the tree. The shortcoming of the new back ref is that it
    only works for pointers in tree blocks referenced by their owner trees.
    
    This is mostly a problem for snapshots, where resolving one of these
    fuzzy back references would be O(number_of_snapshots) and quite slow.
    The solution used here is to use the fuzzy back references in the common
    case where a given tree block is only referenced by one root,
    and use the full back references when multiple roots have a reference
    on a given block.
    
    This commit adds per subvolume red-black tree to keep trace of cached
    inodes. The red-black tree helps the balancing code to find cached
    inodes whose inode numbers within a given range.
    
    This commit improves the balancing code by introducing several data
    structures to keep the state of balancing. The most important one
    is the back ref cache. It caches how the upper level tree blocks are
    referenced. This greatly reduce the overhead of checking back ref.
    
    The improved balancing code scales significantly better with a large
    number of snapshots.
    
    This is a very large commit and was written in a number of
    pieces.  But, they depend heavily on the disk format change and were
    squashed together to make sure git bisect didn't end up in a
    bad state wrt space balancing or the format change.
    
    Signed-off-by: default avatarYan Zheng <zheng.yan@oracle.com>
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    5d4f98a2