ext2 sparse superblocks

[Michael Rothwell]

I'm looking for documentation on Ext2's support for sparse superblocks.
Canvasing search engines gets me the same two references to tune2fs and
the dac960. I've also looked in /usr/doc and
/usr/src/linux/Documentation without success.

What it the method uses to reduce the number of superblocks? How are
they laid out before vs after sparse_super is enabled? Any caveats?

Thanks.

-M