• Darrick J. Wong's avatar
    xfs: set up per-AG free space reservations · 3fd129b6
    Darrick J. Wong authored
    One unfortunate quirk of the reference count and reverse mapping
    btrees -- they can expand in size when blocks are written to *other*
    allocation groups if, say, one large extent becomes a lot of tiny
    extents.  Since we don't want to start throwing errors in the middle
    of CoWing, we need to reserve some blocks to handle future expansion.
    The transaction block reservation counters aren't sufficient here
    because we have to have a reserve of blocks in every AG, not just
    somewhere in the filesystem.
    
    Therefore, create two per-AG block reservation pools.  One feeds the
    AGFL so that rmapbt expansion always succeeds, and the other feeds all
    other metadata so that refcountbt expansion never fails.
    
    Use the count of how many reserved blocks we need to have on hand to
    create a virtual reservation in the AG.  Through selective clamping of
    the maximum length of allocation requests and of the length of the
    longest free extent, we can make it look like there's less free space
    in the AG unless the reservation owner is asking for blocks.
    
    In other words, play some accounting tricks in-core to make sure that
    we always have blocks available.  On the plus side, there's nothing to
    clean up if we crash, which is contrast to the strategy that the rough
    draft used (actually removing extents from the freespace btrees).
    Signed-off-by: 's avatarDarrick J. Wong <darrick.wong@oracle.com>
    Reviewed-by: 's avatarDave Chinner <dchinner@redhat.com>
    Signed-off-by: 's avatarDave Chinner <david@fromorbit.com>
    3fd129b6