Chapter 5

CHAPTER 5Managing Control Files, Online Redo Logs, and Archiving

Introduction• An Oracle database consists of three types of mandatory

files: datafiles, control files, and online redo logs.• Chapter 4 focused on tablespaces and datafiles.• This chapter focuses on managing control files, online

redo logs, and archiving

Control File Contents• Database name• Names and locations of datafiles• Names and locations of online redo log files• Current online redo log sequence number• Checkpoint information• Names and locations of Oracle Recovery Manager

(RMAN) backup files (if using)

Displaying the Contents of a Control File

SQL> oradebug setmypidSQL> oradebug unlimitSQL> alter session set events 'immediate trace name controlf level 9';SQL> oradebug tracefile_name

Viewing Control File Names and Locations

SQL> show parameter control_filesSQL> select name from v$controlfile;

Adding a Control File1. Alter the initialization file CONTROL_FILES parameter

to include the new location and name of the control file.2. Shut down your database.3. Use an OS command to copy an existing control file to

the new location and name.4. Restart your database.

Moving a Control File1. Determine the CONTROL_FILES parameter’s current

value2. Alter your CONTROL_FILES parameter to reflect that

you’re moving a control file3. Shut down your database:4. At the OS prompt, move the control file to the new

location.5. Start up your database

Removing a Control File1. Identify which control file has experienced media failure

by inspecting the alert.log for information.2. Remove the unavailable control file name from the

CONTROL_FILES parameter. If you’re using an init.ora file, modify the file directly with an OS editor (such as vi). If you’re using an spfile, modify the CONTROL_FILES parameter with the ALTER SYSTEM statement.

3. Stop and start your database.

Online Redo Logs• Online redo logs store a record of transactions that have

occurred in your database• These logs provide a mechanism for you to recover your

database in the event of a failure• You’re required to have at least two online redo-log

groups in your database• Each online redo log group must contain at least one

online redo-log member• The member is the physical file that exists on disk• You can create multiple members in each redo log group,

which is known as multiplexing your online redo log group

Online Redo Log Group Considerations

• Create at least three online redo log groups with two members in each group

• Always create online redo log groups with the same physical size of file

Online Redo Log Configuration

Protecting Online Redo Logs• Multiplex groups to have multiple members.• Never allow two members of the same group to share the

same controller.• Never put two members of the same group on the same

physical disk.• Ensure that OS file permissions are set appropriately.• Use physical storage devices that are redundant (that is,

RAID).• Appropriately size the log files so that they switch and are

archived at regular intervals.• Set the ARCHIVE_LAG_TARGET initialization parameter to

ensure that the online redo logs are switched at regular intervals.

Displaying Online Redo-Log Information

select a.group#,a.member,b.status,b.archived,bytes/1024/1024 mbytesfrom v$logfile a, v$log bwhere a.group# = b.group#order by 1,2;

Determining the Optimal Size of Online Redo-Log Groups

select count(*),to_char(first_time,'YYYY:MM:DD:HH24')from v$log_historygroup by to_char(first_time,'YYYY:MM:DD:HH24')order by 2;

SQL> select optimal_logfile_size from v$instance_recovery;

Avoiding Delays in Switching Online Redo Logs

• Add more redo-log groups• Lower the value of FAST_START_MTTR_TARGET. Doing

so causes the database-writer process to write older modified blocks to disk in a smaller timeframe

• Tune the database-writer process (modify DB_WRITER_PROCESSES)

Adding Online Redo-Log Groupsalter database add logfile group 5('/ora01/oraredo/O11R2/redo05a.rdo', '/ora02/oraredo/O11R2/redo05b.rdo') SIZE 500M;

•Lower the value of FAST_START_MTTR_TARGET. Doing so causes the database-writer process to write older modified blocks to disk in a smaller timeframe.

Resizing Online Redo-Log Groups• Add log groups with files sized appropriately• Drop the old log groups

Adding Online Redo-Log Files to a GroupSQL> alter database add logfile member '/ora02/oraredo/O11R2/redo04c.rdo'to group 4;

Removing Online Redo-Log Files from a Group

SELECT a.group#, a.member, b.status, b.archived, SUM(b.bytes)/1024/1024 mbytesFROM v$logfile a, v$log bWHERE a.group# = b.group#GROUP BY a.group#, a.member, b.status, b.archivedORDER BY 1, 2

SQL> alter database drop logfile member '/ora02/oraredo/O11R2/redo04a.rdo';

Moving or Renaming Redo-Log Files• You can add new log files in the new location and drop the

old log filesor• Physically move the files and update the control file with

the new location:

1. Shut down your database2. From the OS prompt, move the files3. Start up your database in mount mode4. Update the control file with the new file locations and

names via the ALTER DATABASE statement5. Open the database

Archive Redo Log Architectural Decisions

• Where to place the archive redo logs and whether to use the fast recovery area to store them

• How to name the archive redo logs• How much space to allocate to the archive redo log location• How often to back up the archive redo logs• When it’s okay to permanently remove archive redo logs from

disk• How to remove archive redo logs (e.g., have RMAN remove

the logs, based on a retention policy)• Whether multiple archive redo log locations should be enabled• (When to schedule the small amount of downtime that’s

required (if a production database)

Enabling Archive Log Mode$ sqlplus / as sysdbaSQL> shutdown immediate;SQL> startup mount;SQL> alter database archivelog;SQL> alter database open;

Note: You can confirm archivelog mode with this query:SQL> archive log list;

Summary• This chapter describes how to configure and manage

control files and online redo log files.• Control files and online redo logs are critical database

files.• These files are critical because a normally operating

database cannot function without these files.• Control files are small binary files that contain information

regarding the structure of the database.• Online redo logs are crucial files that record the

transaction history of the database.