ORACLE vs Berkeley DB: shared memory

I wrote about my experience when I copied Berkeley DB(aka: SleepyCat) data from AMD64 machine to x86 32-bit machine and I needed to remove "__db.001" file to start it up in my past blog. I ran into another occasion where I needed to remove "__db.\*" files to start up Sun Java System Calendar Server.

So, I wondered what are these "__db.\*" files and found this.

Shared memory regions
system memory used by Berkeley DB is potentially useful past the lifetime of any particular process. Therefore, additional cleanup may be necessary after an application fails because there may be no way for Berkeley DB to ensure that system resources backing the shared memory regions are returned to the system.


I think the action of removing "__db.\*" files corresponds to the excerpt above.

Incidentally, this reminded me of ipcrm usage after oracle db crash. Sometimes, oracle db will not restart unless OS admin ipcrm's shared memory. Below, you can see shared memory was allocated on startup by sysdba.

$ ipcs -msp
IPC status from  as of Thu Jan 28 16:54:59 JST 2010
T         ID      KEY        MODE        OWNER    GROUP  CPID  LPID
Shared Memory:
m        102   0xf007cd34 --rw-r-----   ora10g oinstall  5503 29721
Semaphores:
s          7   0xe914e1e0 --ra-r-----   ora10g oinstall

$ tail -12 $ORACLE_HOME/admin/$ORACLE_SID/adump/ora_5503.aud
Redo thread mounted by this instance: 0 
Oracle process number: 0
Unix process pid: 5503, image: oracle@eslab23

Mon Dec 21 15:23:19 2009
ACTION : 'STARTUP'
DATABASE USER: '/'
PRIVILEGE : SYSDBA
CLIENT USER: ora10g
CLIENT TERMINAL: Not Available


I don't know if post-mortem analysis of this shared memory debris is possible in Oracle RDBMS case.
In BDB case, "__db.\*" files contain some info. I hope I can write about it in this blog soon.
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