Download and members

7
Maintaining Redo Log Files
Objectives
• Explaining the use of online redo log
files
• Obtaining log and archive information
• Controlling log switches and
checkpoints
• Multiplexing and maintaining online
redo log files
• Planning online redo log files
• Troubleshooting common redo log file
problems
7-2
Using Redo Log Files
• Record all
Control
files
changes made
to the database
Data files
Database
7-3
• Used only for
Redo log
files
recovery
Introduction to Online
Redo Log Files
Components:
•Redo log group
•Online redo log
file or member
•Archived redo
log file
•ARCn
7-4
Introduction to Online
Redo Log Files
Components:
•Redo log buffer
•LGWR
•CKPT
7-5
Introduction to Online
Redo Log Files
File 1 is written to
File A by the
ARCn process
after the log
switch is
complete
7-6
Introduction to Online
Redo Log Files
File 1 and File 3
are written to by
the LGWR
process
simultaneously
until both are full
7-7
Redo Log Groups and Members
Group 1
Group 2
Member
Member
Group 3
Member
Member
Member
Member
Min. 2 groups
Min. 1 and up to 5 members/group
7-8
Disk 1
Disk 2
Redo Log Groups and Members
•LGWR concurrently writes to all
members of a group
•Log Sequence numbers are written to the
group
•Log Sequence number is assigned to a
group by Oracle each time: unique id
•Current log number is stored in control
file
7-9
Oracle Architecture
Instance
SGA
Database
buffer
cache
DBWR
Redo log
buffer
CKPT
LGWR
ARCH
Control
files
Parameter
file
Data files
Password
file
Redo log
files
Database
7-10
Archived
log files
LGWR, Log Switches and Checkpoints
LGWR writes from buffer to file when:
•A transaction commits
•Redo buffer is one-third full
•>1MB of changed records
•Timeout occurs (every 3 secs)
•Before DBWR writes buffer to data files
7-11
LGWR, Log Switches and Checkpoints
•Log switch: event when LGWR switches
to a new group; Oracle then assigns a
new sequence number
•DBA can force a switch
Checkpoint:
•Dirty DB buffers written to data files
•Updates headers of data and control files
•Occurs when
• Every log switch
• Instance shuts down
• Manually by DBA
7-12
Without Archiving
Backup
Data files
50
7-13
Control
files
51
t1
Disk failure
100
t2
101
With Archiving
Backup
Archived
redo logs
Disk failure
50
Data files
Control
files
50
51
t1
7-14
99
100
t2
101
To Archive Or Not?
Oracle can be configured to operate in:
• ARCHIVELOG or NOARCHIVELOG mode
Two ways to archive redo log files:
• Manually or Automatically
LOG_ARCHIVE_START parameter
indicates if manual or automatic
7-15
Obtaining Information
About Archiving
• Server Manager command:
SQL>ARCHIVE LOG LIST;
• V$DATABASE:
– NAME
(select name, log_mode from v$databse)
– LOG_MODE
• V$INSTANCE
– ARCHIVER(select archiver from v$instance)
7-16
Obtaining Information
About Groups
V$THREAD:
• GROUPS
• CURRENT_GROUP#
• SEQUENCE#
select groups, current_group#, sequence#
from v$thread;
7-17
Obtaining Information About
Groups and Members
V$LOG:
• GROUP#
• MEMBERS
• STATUS
• SEQUENCE#
• BYTES
select group#, sequence#, bytes, members, status
from v$log;
7-18
Obtaining Information About
Groups and Members
V$LOGFILE:
• GROUP#
• STATUS
• MEMBER
select * from v$logfile;
STATUS for groups(G) and members (M):
UNUSED(G): never been written to
CURRENT(G): current group
ACTIVE(G): active but not current; needed for recovery
INACTIVE(G): no longer needed for recovery
INVALID(M): file is inaccessible
BLANK(M): file is in use
7-19
Log Switches and Checkpoints
• Force log switches with the command:
SQL> ALTER SYSTEM SWITCH LOGFILE;
• Control checkpoints with the
initialization parameters:
– LOG_CHECKPOINT_INTERVAL
– LOG_CHECKPOINT_TIMEOUT
SQL> ALTER SYSTEM CHECKPOINT
7-20
Adding Online Redo Log Groups
ALTER DATABASE ADD LOGFILE
(‘/DISK3/log3a.rdo’,
‘/DISK4/log3b.rdo’) size 1M;
log1a.rdo
log1b.rdo
Group 1
7-21
log2b.rdo
Group 2
log3a.rdo
log3b.rdo
Group 3
Adding Online Redo
Log Members
ALTER DATABASE ADD LOGFILE MEMBER
‘/DISK4/log1b.rdo’ TO GROUP 1,
‘/DISK4/log2b.rdo’ TO GROUP 2;
log1a.rdo
log1b.rdo
Group 1
7-22
log2a.rdo
log2b.rdo
Group 2
How to Relocate
Online Redo Log Files
1. Shut down the database.
2. Copy the online redo log files to the
new location.
3. Mount the database.
4. Execute the ALTER DATABASE
RENAME FILE command.
ALTER DATBASE RENAME FILE ‘fname’ TO ‘fname’
5. Open the database.
7-23
Dropping Online Redo
Log Groups
ALTER DATABASE DROP LOGFILE
GROUP 3;
log1a.rdo
log1b.rdo
Group 1
7-24
log2a.rdo
log3a.rdo
log3b.rdo
log2b.rdo
Group 2
Group 3
Dropping Online Redo
Log Members
ALTER DATABASE DROP LOGFILE MEMBER
‘/DISK4/log2b.dbf’;
log1a.rdo
log1b.rdo
Group 1
7-25
log2a.rdo log2b.rdo
Group 2
Clearing Online Redo Log Files
If it gets corrupted.
Example
ALTER DATABASE CLEAR LOGFILE
‘/DISK3/log2a.rdo’;
7-26
Online Redo Log Configuration
Group 1
Group 2
Member
Member
Group 3
Member
Member
Member
Disk 1
7-27
Disk 2
Disk 3
Possible LGWR Errors
• One member of a group of two or more
is not available.
• All members of the next group are not
available.
• All members of the current group are
not available.
7-28
OMF and Log Files
•You can create as many redo log groups as you
need, bounded by MAXLOGFILES setting (when
database was created).
•You can multiplex each of those groups with up
to five additional OMF members (bounded by the
MAXLOGMEMBERS). D
•Different redo log group members are created in
different locations, as defined by parameters
such as db_create_online_log_dest_n
7-29
OMF and Log Files (cont.)
Examples
•ALTER DATABASE ADD LOGFILE; or
•ALTER DATABASE ADD LOGFILE GROUP 3 SIZE
300M; - to add a log group
•ALTER DATABASE DROP LOGFILE GROUP 1; to drop a log group (may not be current log
group)
•Not possible to add an additional log group
member that is an OMF
•Can drop an OMF redo log member:ALTER
DATABASE DROP LOGFILE MEMBER.... Oracle
will remove the dropped redo log member.
7-30
OMF and Log Files (cont.)
Example Create Database Command:
CREATE DATABASE mydb
DATAFILE SIZE 500M
LOGFILE
GROUP 1 SIZE 10M , GROUP 2 SIZE 10M
DEFAULT TEMPORARY TABLESPACE temp
TEMPFILE SIZE 100M
UNDO TABLESPACE undotbs1
DATAFILE SIZE 50M
MAXLOGFILES = 5 MAXLOGMEMBERS = 5
MAXDATAFILES = 600
NOARCHIVELOG;
7-31
Summary
• Controlling log switches and
checkpoints
• Administering online redo log files
7-32