Cause: DB_BLOCK_SIZE initialization parameter is wrong for the database being mounted. It does not match the value used to create the database.
Action: Fix the value of the DB_BLOCK_SIZE parameter or mount a database that matches the value.
Example:
SQL> startup
ORACLE instance started.
Total System Global Area 520093696 bytes
Fixed Size 2031352 bytes
Variable Size 176161032 bytes
Database Buffers 339738624 bytes
Redo Buffers 2162688 bytes
ORA-00058: DB_BLOCK_SIZE must be 16384 to mount this database (not 8192)
SQL> shutdown immediate
ORA-01507: database not mounted
ORACLE instance shut down.
SQL> exit
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