How are alternate superblocks repaired?

[Theodore Tso]

On Tue, Oct 02, 2007 at 03:38:47PM -0400, Thomas Watt wrote:
> In case you are interested, here is link to a web page on Structure Marking:
> http://www.multicians.org/thvv/marking.html

I actually have used a Multics system way back when (I was actually
logged into MIT Multics when it was finally shutdown[1]).  The com_err
library and the ss library in e2fsprogs was largely inspired from
Multics, and I do use structure magic numbers in memory to protect
against programming errors, which is basically a very simple structure
marking technique.

I'm a bit dubious about how useful simply structure matching would be
for modern Linux systems, since a large number of errors really are
silent bit flips in the data, that wouldn't be detected simply by
checking the expected structure ID at the beginning of the on-disk
object.  We are planning on adding checksum to metadata for ext4,
which will help a lot in terms of detected bad metadata.

Regards,   ("You are protected from preemption"  :-)

[1]  http://stuff.mit.edu/afs/sipb/project/eichin/sipbscan/

					- Ted