linux-ext4 - ext4 backup super block update frequency

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Message-ID: <CAPLK-i8LrHsLU2b4KEwjVe4EgLRyL=gfyMVVFuiVS_HGjBxbmQ@mail.gmail.com>
Date:   Fri, 31 Aug 2018 12:40:33 -0400
From:   Shehbaz Jaffer <shehbazjaffer007@...il.com>
To:     linux-ext4@...r.kernel.org
Subject: ext4 backup super block update frequency

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