• Arne Jansen's avatar
    btrfs: quasi-round-robin for chunk allocation · 73c5de00
    Arne Jansen authored
    In a multi device setup, the chunk allocator currently always allocates
    chunks on the devices in the same order. This leads to a very uneven
    distribution, especially with RAID1 or RAID10 and an uneven number of
    devices.
    This patch always sorts the devices before allocating, and allocates the
    stripes on the devices with the most available space, as long as there
    is enough space available. In a low space situation, it first tries to
    maximize striping.
    The patch also simplifies the allocator and reduces the checks for
    corner cases.
    The simplification is done by several means. First, it defines the
    properties of each RAID type upfront. These properties are used afterwards
    instead of differentiating cases in several places.
    Second, the old allocator defined a minimum stripe size for each block
    group type, tried to find a large enough chunk, and if this fails just
    allocates a smaller one. This is now done in one step. The largest possible
    chunk (up to max_chunk_size) is searched and allocated.
    Because we now have only one pass, the allocation of the map (struct
    map_lookup) is moved down to the point where the number of stripes is
    already known. This way we avoid reallocation of the map.
    We still avoid allocating stripes that are not a multiple of STRIPE_SIZE.
    73c5de00
volumes.h 5.92 KB