• NeilBrown's avatar
    md/raid10: add reshape support · 3ea7daa5
    NeilBrown authored
    A 'near' or 'offset' lay RAID10 array can be reshaped to a different
    'near' or 'offset' layout, a different chunk size, and a different
    number of devices.
    However the number of copies cannot change.
    Unlike RAID5/6, we do not support having user-space backup data that
    is being relocated during a 'critical section'.  Rather, the
    data_offset of each device must change so that when writing any block
    to a new location, it will not over-write any data that is still
    This means that RAID10 reshape is not supportable on v0.90 metadata.
    The different between the old data_offset and the new_offset must be
    at least the larger of the chunksize multiplied by offset copies of
    each of the old and new layout. (for 'near' mode, offset_copies == 1).
    A larger difference of around 64M seems useful for in-place reshapes
    as more data can be moved between metadata updates.
    Very large differences (e.g. 512M) seem to slow the process down due
    to lots of long seeks (on oldish consumer graded devices at least).
    Metadata needs to be updated whenever the place we are about to write
    to is considered - by the current metadata - to still contain data in
    the old layout.
    [unbalanced locking fix from Dan Carpenter <dan.carpenter@oracle.com>]
    Signed-off-by: default avatarNeilBrown <neilb@suse.de>
raid10.h 4.33 KB