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    md: Allow devices to be re-added to a read-only array. · 7ceb17e8
    NeilBrown 提交于
    When assembling an array incrementally we might want to make
    it device available when "enough" devices are present, but maybe
    not "all" devices are present.
    If the remaining devices appear before the array is actually used,
    they should be added transparently.
    
    We do this by using the "read-auto" mode where the array acts like
    it is read-only until a write request arrives.
    
    Current an add-device request switches a read-auto array to active.
    This means that only one device can be added after the array is first
    made read-auto.  This isn't a problem for RAID5, but is not ideal for
    RAID6 or RAID10.
    Also we don't really want to switch the array to read-auto at all
    when re-adding a device as this doesn't really imply any change.
    
    So:
     - remove the "md_update_sb()" call from add_new_disk().  This isn't
       really needed as just adding a disk doesn't require a metadata
       update.  Instead, just set MD_CHANGE_DEVS.  This will effect a
       metadata update soon enough, once the array is not read-only.
    
     - Allow the ADD_NEW_DISK ioctl to succeed without activating a
       read-auto array, providing the MD_DISK_SYNC flag is set.
       In this case, the device will be rejected if it cannot be added
       with the correct device number, or has an incorrect event count.
    
     - Teach remove_and_add_spares() to be careful about adding spares
       when the array is read-only (or read-mostly) - only add devices
       that are thought to be in-sync, and only do it if the array is
       in-sync itself.
    
     - In md_check_recovery, use remove_and_add_spares in the read-only
       case, rather than open coding just the 'remove' part of it.
    Reported-by: NMartin Wilck <mwilck@arcor.de>
    Signed-off-by: NNeilBrown <neilb@suse.de>
    7ceb17e8
md.c 223.8 KB