Hi All, I am trying to study how backup superblocks are updated and used in the ext4 file system. I create a 2GB file system. This creates 5 backup superblocks with the sparse sb option. I then check the diff of the backup blocks before and after multiple mount, write(2) fsync(2) and close(2) and unmount operations. I can see the primary superblock get updated but I do not see the backup super blocks being updated. My intuition is that the backup blocks are only present so that the recovery can be done by replaying the journal on the backup superblock. Are they updated each time the journal gets full so that the "refreshed" journal can now be replayed on updated backup superblock in case of a crash? If this is incorrect, at what frequency do backup superblocks updated? If we compare this behavior with BtrFS, I can see that for each update on fs tree, the primary block (at offset 64KB) and backup superblock (at 64MB) gets updated. Thanks, Shehbaz
