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The redo log files specified in
the create DATABASE command become thread 1. Additional threads are
created using the ALTER DATABASE ADD LOGFILE command.
For example, to create a thread
while creating the database (a manual process):
MAXLOGFILES 300 MAXLOGMEMBERS 6
MAXDATAFILES 512 MAXINSTANCES 24
DATAFILE '/u01/oracle/data/sys_racdb1' size 1024M
'u02/oracle/redo/file1b') size 50M,
'u02/oracle/redo/file2b') size 50M,
'u02/oracle/redo/file3b') size 50M ;
To add an extra redo log thread:
DATABASE ADD LOGFILE THREAD 2
GROUP 4 ('u01/oracle/redo/file4a',
GROUP 5 ('u01/oracle/redo/file5a',
GROUP 6 ('u01/oracle/redo/file6a',
A thread needs to be enabled
before an instance can use it. While enabling, it can be designated
as a public or a private thread. For example, to enable as a public
DATABASE ENABLE PUBLIC THREAD 2 ;
To enable as a private thread:
DATABASE ENABLE PRIVATE THREAD 3 ;
When a thread is enabled as
private, the thread parameter in the SPFILE (or init.ora) can be
used to associate the instance with a thread. For example, instances
racdb1 and racdb2 will have thread 1 and 2 respectively.
RACDB1.THREAD=1 # RACDB1 instance uses Thread 1
RACDB2.THREAD=2 # RACDB2 instance uses Thread 2
Next, here are some examples of
how to drop a group or member.
For dropping an online redo log
group, use the ALTER DATABASE statement with the DROP LOGFILE
clause. The following statement drops redo log group number 3:
DATABASE DROP LOGFILE GROUP 3;
To drop specific inactive online
redo log members, use the ALTER DATABASE statement with the DROP
LOGFILE MEMBER clause. The following statement drops the redo log
DATABASE DROP LOGFILE
Table 8.8 shows the views that
deal with online redo information.
Displays the redo log file
information from the control file.
Identifies redo log groups
and members and member status.
Contains log history
Table 8.8: Dynamic performance
views related to Redo-Log
Segment Space Management
Each object in the database that
occupies space is associated with a segment. A segment is a set of
extents that contains all the data for a specific logical storage
within a tablespace or multiple tablespaces. An extent is a storage
unit composed of contiguous blocks. The free and used space in a
segment can be managed in two ways:
When a locally managed
tablespace is created using the CREATE TABLESPACE statement, the
SEGMENT SPACE MANAGEMENT clause allows the DBA to specify how free
and used space within a segment is to be managed.
* The Manual Method involves
managing the free space using the free lists of the data blocks.
This method depends on the storage settings like pctused, freelists,
and freelist groups for schema objects. Manual method is the default
parameter when creating a tablespace.
* In the Automatic Method,
Oracle uses bitmaps to manage the free space within segments. A
bitmap, in this case, is a map that describes the status of each
data block within a segment with respect to the amount of space in
the block available for inserting rows. As more space becomes
available or some space gets reduced in a data block, the new amount
of free space is reflected in the bitmap. Automatic segment-space
management is specified at the tablespace level. The Oracle database
server automatically and efficiently manages free and used space
within objects created in such tablespaces.