Sunday, November 9, 2008

Resize the Online Redo Log files

When you are working on large data movement in Oracle, you must concern about some performance overhead and online redo log switch is one of them. Recently i am working on a migration project where i need to migrate a 19.5GB MySQL 5.0 database to ORACLE 10gR2 database. I used Oracle SQL developer to do the job and the job takes about 5 hour to complete. To do the job faster i perform some tuning stuff on ORACLE and increasing the Online Redo Log files is the most fruitful one.

Now i am going to describe how i have done this. The procedure was learned from a release note of metalink .



1. First see the size of the current logs:

SQL> connect / as sysdba

SQL> select group#, bytes, status from v$log;
GROUP# BYTES STATUS
---------- ---------- ----------------
1 52428800 INACTIVE
2 52428800 CURRENT
3 52428800 INACTIVE

Logs are 50MB from above which is default in oracle 10g, let's size them to 100MB.


2. Retrieve all the log member names for the groups:

SQL> select group#, member from v$logfile;

GROUP# MEMBER
--------------- ----------------------------------------
1 /usr/oracle/dbs/log1PROD.dbf
2 /usr/oracle/dbs/log2PROD.dbf
3 /usr/oracle/dbs/log3PROD.dbf


3. Now drop the log group 1 and recreate it with increased size

SQL> alter database drop logfile group 1;

SQL> alter database add logfile group 1
'/usr/oracle/dbs/log1PROD.dbf' size 100M reuse;

4. Check the size of the current logs:

SQL> connect / as sysdba

SQL> select group#, bytes, status from v$log;
GROUP# BYTES STATUS
---------- ---------- ----------------
1 10485760 UNUSED
2 52428800 CURRENT
3 52428800 INACTIVE
5. Do the same for log group 3

SQL> alter database drop logfile group 3;

SQL> alter database add logfile group 3
'/usr/oracle/dbs/log3PROD.dbf' size 100M reuse;


SQL> select group#, bytes, status from v$log;
GROUP# BYTES STATUS
---------- ---------- ----------------
1 10485760 UNUSED
2 52428800 CURRENT
3 10485760 INACTIVE
6. Now we go for group 2 but it is now used by oracle, so first switch the log

SQL> alter system switch logfile;

SQL> select group#, bytes, status from v$log;
GROUP# BYTES STATUS
---------- ---------- ----------------
1 10485760 UNUSED
2 52428800 INACTIVE
3 10485760 CURRENT
SQL> alter database drop logfile group 2;


SQL> alter database add logfile group 2
'/usr/oracle/dbs/log2PROD.dbf' size 100M reuse;

7. Check the size of the current logs:

SQL> connect / as sysdba

SQL> select group#, bytes, status from v$log;
GROUP# BYTES STATUS
---------- ---------- ----------------
1 10485760 UNUSED
2 10485760 UNUSED
3 10485760 CURRENT

** some times you may find that a log group status is ACTIVE, in that case you should
make a database checkpoint like this

SQL> ALTER SYSTEM CHECKPOINT;

** FOR safety reason please take a full backup of your database before performing this.
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