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Command Line Processor for DB2 Client 9.7.2
db2 => db2start
SQL1220N The database manager shared memory set cannot be allocated.
After some searching I discovered that the problem is probably shmmax being too low. A site on tuning Oracle which shows how to increase shmmax:
sudo -s "echo 2147483648 > /proc/sys/kernel/shmmax"
DB2 successfully starts up now.