• Zheng Yan's avatar
    Btrfs: update space balancing code · 1a40e23b
    Zheng Yan authored
    
    
    This patch updates the space balancing code to utilize the new
    backref format.  Before, btrfs-vol -b would break any COW links
    on data blocks or metadata.  This was slow and caused the amount
    of space used to explode if a large number of snapshots were present.
    
    The new code can keeps the sharing of all data extents and
    most of the tree blocks.
    
    To maintain the sharing of data extents, the space balance code uses
    a seperate inode hold data extent pointers, then updates the references
    to point to the new location.
    
    To maintain the sharing of tree blocks, the space balance code uses
    reloc trees to relocate tree blocks in reference counted roots.
    There is one reloc tree for each subvol, and all reloc trees share
    same root key objectid. Reloc trees are snapshots of the latest
    committed roots of subvols (root->commit_root).
    
    To relocate a tree block referenced by a subvol, there are two steps.
    COW the block through subvol's reloc tree, then update block pointer in
    the subvol to point to the new block. Since all reloc trees share
    same root key objectid, doing special handing for tree blocks
    owned by them is easy. Once a tree block has been COWed in one
    reloc tree, we can use the resulting new block directly when the
    same block is required to COW again through other reloc trees.
    In this way, relocated tree blocks are shared between reloc trees,
    so they are also shared between subvols.
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    1a40e23b