SMON: Mark undo segment [segment] as available

Last week I bumped across this error on an Oracle 8i instance following a database restart:

Symptoms:
There are lots of messages appearing in alert log of the following form:

SMON: about to recover undo segment %s
SMON: mark undo segment %s as available

When the recovery is going on after a abnormal shutdown.

Cause:
These errors do not indicate rollback segment corruption.

These messages indicate that there is a problem with the
"rollback_segments" parameter in the init.ora.

Fix:
Check that the rollback segment is included in the "rollback_segments"
parameter then adding the rollback segment to the parameter. If not,
adding the rollback segment and restarting the database will clear up
the problem.

In a few words, what happened was that I have added some rollback segments but forgot to update the init.ora, and the error cropped up when I bounced the database. So, just include the following in the init.ora:

rollback_segments = ( R01, R02, R03, R04, R05, R06, R07, R08, 
R2_01, R2_02, R2_03, R2_04, R2_05, R2_06, R2_07, R2_08 )