When XFS saves secondary Super block

[Ajeet Yadav <ajeet.yadav.77@xxxxxxxxx>]

Often when primary super block is corrupted, and xfs_repair is run on
it, it try to find the secondary super block traversing in block units
from XFS_AG_MIN_BYTES to end of partition.
Now the problem is at first place we do not know whether we actually
have secondary super block or not on partition,
because if we know this inadvance then we may not waste time in
scanning a complete disk.
User point of view is that its like xfs_repair hangs (offcourse
developer knows its not, point of view are different)

1. In which case secondary super block is created, is it created
during format itself, if yes then at what location.
2. In our case we always find that when ever primary super block is
corrupted, xfs_repair never find secondary super block also, so we are
doubt ful of its existence. If its not there then we can just exit
xfs_repair instead of waiting for minutes.