BAD BLOCKS -- What to do?

[Geraldo Veiga]

I had some recurring I/O ERROR messages during boot-up, when mounting
my /usr file system (This is under ISC 2.2).  After running fsck it
reports

BAD BLOCK = <block number>

After running fsck the file system could be mounted without a problem.
After this hapens a couple of times, I decided to add this bad block
number to the bad block list through sysadm.

Questions:

1. Where is this bad block list kept and how does the OS uses it?
2. I this related to the defect list in /etc/partition?