• D
    xfs: preallocate blocks for worst-case btree expansion · 84d69619
    Darrick J. Wong 提交于
    To gracefully handle the situation where a CoW operation turns a
    single refcount extent into a lot of tiny ones and then run out of
    space when a tree split has to happen, use the per-AG reserved block
    pool to pre-allocate all the space we'll ever need for a maximal
    btree.  For a 4K block size, this only costs an overhead of 0.3% of
    available disk space.
    
    When reflink is enabled, we have an unfortunate problem with rmap --
    since we can share a block billions of times, this means that the
    reverse mapping btree can expand basically infinitely.  When an AG is
    so full that there are no free blocks with which to expand the rmapbt,
    the filesystem will shut down hard.
    
    This is rather annoying to the user, so use the AG reservation code to
    reserve a "reasonable" amount of space for rmap.  We'll prevent
    reflinks and CoW operations if we think we're getting close to
    exhausting an AG's free space rather than shutting down, but this
    permanent reservation should be enough for "most" users.  Hopefully.
    Signed-off-by: NDarrick J. Wong <darrick.wong@oracle.com>
    [hch@lst.de: ensure that we invalidate the freed btree buffer]
    Signed-off-by: NChristoph Hellwig <hch@lst.de>
    84d69619
xfs_refcount_btree.h 2.5 KB