• NeilBrown's avatar
    md: update sync_completed and reshape_position even more often. · c03f6a19
    NeilBrown authored
    There are circumstances when a user-space process might need to
    "oversee" a resync/reshape process.  For example when doing an
    in-place reshape of a raid5, it is prudent to take a backup of each
    section before reshaping it as this is the only way to provide
    safety against an unplanned shutdown (i.e. crash/power failure).
    
    The sync_max sysfs value can be used to stop the resync from
    advancing beyond a particular point.
    So user-space can:
      suspend IO to the first section and back it up
      set 'sync_max' to the end of the section
      wait for 'sync_completed' to reach that point
      resume IO on the first section and move on to the next section.
    
    However this process requires the kernel and user-space to run in
    lock-step which could introduce unnecessary delays.
    
    It would be better if a 'double buffered' approach could be used with
    userspace and kernel space working on different sections with the
    'next' section always ready when the 'current' section is finished.
    
    One problem with implementing this is that sync_completed is only
    guaranteed to be updated when the sync process reaches sync_max.
    (it is updated on a time basis at other times, but it is hard to rely
    on that).  This defeats some of the double buffering.
    
    With this patch, sync_completed (and reshape_position) get updated as
    the current position approaches sync_max, so there is room for
    userspace to advance sync_max early without losing updates.
    
    To be precise, sync_completed is updated when the current sync
    position reaches half way between the current value of sync_completed
    and the value of sync_max.  This will usually be a good time for user
    space to update sync_max.
    
    If sync_max does not get updated, the updates to sync_completed
    (together with associated metadata updates) will occur at an
    exponentially increasing frequency which will get unreasonably fast
    (one update every page) immediately before the process hits sync_max
    and stops.  So the update rate will be unreasonably fast only for an
    insignificant period of time.
    Signed-off-by: default avatarNeilBrown <neilb@suse.de>
    c03f6a19
raid5.c 149 KB