The DB2 message SQL20169W The buffer pool is not started appeared in response to a ALTER BUFFERPOOL statement
db2 ALTER BUFFERPOOL ITMBUF8K IMMEDIATE size 250000 ---SQL20169W The buffer pool is not started. SQLSTATE=01654
To overcome this problem , disconnect all the applications connected to the database, and activate the database.
There are a number of methods to disconnect all the applications , such as DB2 deactivate database