nilfs2: introduce secondary super block

The former versions didn't have extra super blocks.  This improves the
weak point by introducing another super block at unused region in tail of
the partition.

This doesn't break disk format compatibility; older versions just ingore
the secondary super block, and new versions just recover it if it doesn't
exist.  The partition created by an old mkfs may not have unused region,
but in that case, the secondary super block will not be added.

This doesn't make more redundant copies of the super block; it is a future
work.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 0 insertions, 2 deletions

diff --git a/fs/nilfs2/segment.h b/fs/nilfs2/segment.h
index 4a64eb82f1f5..a98fc1ed0bbb 100644
--- a/fs/nilfs2/segment.h
+++ b/fs/nilfs2/segment.h
@@ -206,8 +206,6 @@ enum {
                                            logical segment with a super root */
 #define NILFS_SC_DEFAULT_SR_FREQ    30  /* Maximum frequency of super root
                                            creation */
-#define NILFS_SC_DEFAULT_SB_FREQ    30  /* Minimum interval of periodical
-                                           update of superblock (reserved) */
 
 /*
  * The default threshold amount of data, in block counts.