What is High Water Mark in Oracle?

1. High water mark is the maximum amount of database blocks used so far by a segment. This mark cannot be reset by delete operations.

2. Delete Table operation won't reset HWM.

3. TRUNCATE will reset HWM.

4. The high water mark level is just a line separate the used blocks and free blocks.

The blocks above the HWM level is free blocks, they are ready to use.
The blocks below the HWM level is used blocks, they are already used.

I hope now you understand well. see the below example.

for example, if you delete some huge records from the database, that data will delete but the blocks are not ready to used, because that blocks are still below HWM level, so delete command never reset the HWM level,

At the same time you truncate the date, then the data will delete and that used blocks will goto above the HWM level, now its ready to used. now they consider has free blocks.

OC4J Configuration issue… OC4J_DBConsole_ not found.

I am trying to start EM on my newly created database, while starting EM, getting failed with OC4J Configuration issue… while diagnosed there is no OC4J_DBConsole_localhost directory exist in ORACLE_HOME. Means this may be the case where Database Control not configured while database created OR misconfigured.

 Error log:

 [oracle@localhost ~]$ emctl status dbconsole

OC4J Configuration issue. /u01/app/oracle/product/11.2.0/dbhome_1/oc4j/j2ee/OC4J_DBConsole_localhost.localhost.com_fmw not found.
[oracle@localhost ~]$ emctl start dbconsole
OC4J Configuration issue. /u01/app/oracle/product/11.2.0/dbhome_1/oc4j/j2ee/OC4J_DBConsole_localhost.localhost.com_fmw not found.
This is due to a misconfiguration of dbconsole during database setup. The fastest solution is usually to recreate repository and configuration.

 Solution: is to Configure EM with the help of following “Enterprise Manager Configuration Assistant” (i.e. emca) utility from Oracle user Before that:

1.      Ensure environment variable properly set like below.

2.      Ensure your listener properly configured and started.

3.      Ensure password file is created. (Optional)

4.      You need SYS,SYSMAN & DBSNMP password.

5.      Ensure your /etc/hosts file mentioned below two lines.

 Environment Variable

[oracle@localhost~]$ export ORACLE_SID=orcl

[oracle@localhost~]$ export ORACLE_UNQNAME=orcl

[oracle@localhost~]$ export ORACLE_HOME=/home/oracle/app/oracle/product/11.2.0/dbhome_1/

[oracle@localhost~]$ export PATH=$PATH:/home/oracle/app/oracle/product/11.2.0/dbhome_1/bin/

 First, deconfigure dbconsole:

[oracle@localhost ~]$ emca -deconfig dbcontrol db -repos drop
STARTED EMCA at Aug 5, 2013 7:54:35 PM
EM Configuration Assistant, Version 11.2.0.0.2 Production
Copyright (c) 2003, 2005, Oracle.  All rights reserved.

Enter the following information:
Database SID: fmw
Listener port number: 1521
Password for SYS user:
Password for SYSMAN user:

Do you wish to continue? [yes(Y)/no(N)]: Y
Aug 5, 2013 7:54:54 PM oracle.sysman.emcp.EMConfig perform
INFO: This operation is being logged at /u01/app/oracle/cfgtoollogs/emca/fmw/emca_2013_08_05_19_54_34.log.
Aug 5, 2013 7:54:55 PM oracle.sysman.emcp.EMDBPreConfig performDeconfiguration
WARNING: EM is not configured for this database. No EM-specific actions can be performed.
Aug 5, 2013 7:54:55 PM oracle.sysman.emcp.EMReposConfig invoke
INFO: Dropping the EM repository (this may take a while) ...
Aug 5, 2013 7:57:04 PM oracle.sysman.emcp.EMReposConfig invoke
INFO: Repository successfully dropped
Enterprise Manager configuration completed successfully
FINISHED EMCA at Aug 5, 2013 7:57:04 PM

Now, let's configure it again

[oracle@localhost ~]$ emca -config dbcontrol db -repos create
STARTED EMCA at Aug 5, 2013 7:58:22 PM
EM Configuration Assistant, Version 11.2.0.0.2 Production
Copyright (c) 2003, 2005, Oracle.  All rights reserved.

Enter the following information:
Database SID: fmw
Listener port number: 1521
Listener ORACLE_HOME [ /u01/app/oracle/product/11.2.0/dbhome_1 ]:
Password for SYS user:
Password for DBSNMP user:
Password for SYSMAN user:
Email address for notifications (optional):
Outgoing Mail (SMTP) server for notifications (optional):
-----------------------------------------------------------------

You have specified the following settings
Database ORACLE_HOME ................ /u01/app/oracle/product/11.2.0/dbhome_1
Local hostname ................ localhost.localhost.com
Listener ORACLE_HOME ................ /u01/app/oracle/product/11.2.0/dbhome_1
Listener port number ................ 1521
Database SID ................ fmw
Email address for notifications ...............
Outgoing Mail (SMTP) server for notifications ...............
-----------------------------------------------------------------

Do you wish to continue? [yes(Y)/no(N)]: Y
Aug 5, 2013 7:58:53 PM oracle.sysman.emcp.EMConfig perform
INFO: This operation is being logged at /u01/app/oracle/cfgtoollogs/emca/fmw/emca_2013_08_05_19_58_21.log.
Aug 5, 2013 7:58:54 PM oracle.sysman.emcp.EMReposConfig createRepository
INFO: Creating the EM repository (this may take a while) ...
Aug 5, 2013 8:05:56 PM oracle.sysman.emcp.EMReposConfig invoke
INFO: Repository successfully created
Aug 5, 2013 8:06:02 PM oracle.sysman.emcp.EMReposConfig uploadConfigDataToRepository
INFO: Uploading configuration data to EM repository (this may take a while) ...
Aug 5, 2013 8:07:14 PM oracle.sysman.emcp.EMReposConfig invoke
INFO: Uploaded configuration data successfully
Aug 5, 2013 8:07:17 PM oracle.sysman.emcp.util.DBControlUtil configureSoftwareLib
INFO: Software library configured successfully.
Aug 5, 2013 8:07:17 PM oracle.sysman.emcp.EMDBPostConfig configureSoftwareLibrary
INFO: Deploying Provisioning archives ...
Aug 5, 2013 8:08:07 PM oracle.sysman.emcp.EMDBPostConfig configureSoftwareLibrary
INFO: Provisioning archives deployed successfully.
Aug 5, 2013 8:08:07 PM oracle.sysman.emcp.util.DBControlUtil secureDBConsole
INFO: Securing Database Control (this may take a while) ...
Aug 5, 2013 8:08:53 PM oracle.sysman.emcp.util.DBControlUtil secureDBConsole
INFO: Database Control secured successfully.
Aug 5, 2013 8:08:53 PM oracle.sysman.emcp.util.DBControlUtil startOMS
INFO: Starting Database Control (this may take a while) ...
Aug 5, 2013 8:10:16 PM oracle.sysman.emcp.EMDBPostConfig performConfiguration
INFO: Database Control started successfully
Aug 5, 2013 8:10:17 PM oracle.sysman.emcp.EMDBPostConfig performConfiguration
INFO: >>>>>>>>>>> The Database Control URL is https://localhost.localhost.com:5500/em <<<<<<<<<<<
Aug 5, 2013 8:10:28 PM oracle.sysman.emcp.EMDBPostConfig invoke
WARNING:
************************ WARNING  ************************

Management Repository has been placed in secure mode wherein Enterprise Manager data will be encrypted.  The encryption key has been placed in the file: /u01/app/oracle/product/11.2.0/dbhome_1/localhost.localhost.com_fmw/sysman/config/emkey.ora.   Please ensure this file is backed up as the encrypted data will become unusable if this file is lost.

***********************************************************
Enterprise Manager Configuration completed successfully
FINISHED EMCA at Aug 5, 2013 8:10:28 PM

To access your Enterprise Manager, Browse URL: https://localhost.localdomain:5500/em.