Low or Poor Buffer Cache Hit Ratio and Full Table Scans

 Normally DBAs do not much feel bothered by poor buffer cache hit ratio especially in data warehouse environments where full table scans may be quire frequent and acceptable. However, if you have OLTP environment, I still believe that buffer cache hit ratio should be taken care of and DBAs should try to keep it as high as they can. In OLTP environments, full table scans would cause cache hit ratio to be low because frequent full table scans of different tables would cause entire tables to be read in to buffer cache even if only a few block are needed for processing.

ORA-16012: database identifier mismatch

If this error message is appearing in the alert log file, most of the time it is related to log_archive_dest_1 for standby database’s local archive generation and/or log_archive_dest_2, that points to the primary database. You should check if every parameter is set properly and TNS string used to point to the primary database is set properly. For further information, you can see this article that explains how to properly configure dataguard.

PX Deq: Signal ACK RSG

PX Deq: Signal ACK RSG wait event does not have much information available even on My Oracle Support portal. I was not able to find much information about this. I noticed this wait event at the top of wait events in my AWR report as can be seen bellow that made me do some investigation. First and the foremost, this wait event is related to parallel execution of SQLs.

ORA-15073: diskgroup DATA is mounted by another ASM instance

SQL> drop diskgroup data; drop diskgroup data * ERROR at line 1: ORA-15039: diskgroup not dropped ORA-15073: diskgroup DATA is mounted by another ASM instance

If you are not able to drop a diskgroup, and ORA-15039 is returned along with ORA-15039: diskgroup not dropped, it means the diskgroup needs to be dismounted on all instances except current instance where you are issuing DROP DISKGROUP command.