md: support 'external' metadata for md arrays

- Add a state flag 'external' to indicate that the metadata is managed
  externally (by user-space) so important changes need to be
  left of user-space to handle.
  Alternates are non-persistant ('none') where there is no stable metadata -
  after the  array is stopped there is no record of it's status - and
  internal which can be version 0.90 or version 1.x
  These are selected by writing to the 'metadata' attribute.

- move the updating of superblocks (sync_sbs) to after we have checked if
  there are any superblocks or not.

- New array state 'write_pending'.  This means that the metadata records
  the array as 'clean', but a write has been requested, so the metadata has
  to be updated to record a 'dirty' array before the write can continue.
  This change is reported to md by writing 'active' to the array_state
  attribute.

- tidy up marking of sb_dirty:
   - don't set sb_dirty when resync finishes as md_check_recovery
     calls md_update_sb when the sync thread finishes anyway.
   - Don't set sb_dirty in multipath_run as the array might not be dirty.
   - don't mark superblock dirty when switching to 'clean' if there
     is no internal superblock (if external, userspace can choose to
     update the superblock whenever it chooses to).

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/raid/md_k.h b/include/linux/raid/md_k.h
index dcb729244f4..b579cc62830 100644
--- a/include/linux/raid/md_k.h
+++ b/include/linux/raid/md_k.h
@@ -130,6 +130,9 @@ struct mddev_s
                                         minor_version,
                                         patch_version;
         int                             persistent;
+        int                             external;       /* metadata is
+                                                         * managed externally */
+        char                            metadata_type[17]; /* externally set*/
         int                             chunk_size;
         time_t                          ctime, utime;
         int                             level, layout;