What happens when the database is in
Begin backup mode?
When a
hot backup runs in the oracle, first we will make the database in begin backup
mode and then we copy the database files to tape or disk. The question that
most of us will have is: what happens to the datafiles when the database is in begin backup mode?
Offline?
Or frozen? Answer is No.
“Some of the Oracle books contains that the
database files are not writable during the backup mode and the changes are
stored in the SGA, redo logs and Rollback segments. And these changes will be
the written into the data files when the database is taken out of begin backup
mode. “
But the
above predictions are opposed. It does not stop writing to the data files. The
database almost works in normal way when the database in the backup mode. It
can be summarized in as follows:
·
DBWn process
writes out all the dirty blocks of a
particular SCN, i.e. when a check point process runs, the DBWn process writes all
the blocks related to that SCN generated.
·
CKPT
process stops updating the checkpoint SCN field in the data files headers
and begins updating the Hot backup
checkpoint SCN field in the data files headers.
·
LGWR
process begins logging of the changed blocks.
By freezing the checkpoint SCN in the data file headers, any subsequent
recovery on that particular hot backup will know that it must commence at that
SCN. Having an old SCN in the file header tells
recovery that the file is an old one, and that it should look for the
archivelog containing that SCN, and applies recovery starting there.
Here we need to know that in hot backup mode the checkpoints
to data files are not suppressed. The main checkpoint flag is frozen, but the hot backup Checkpoint SCN
will be updated in the file header.
SQL> create table student(name
varchar(20));
Table created.
SQL> select * from tab;
TNAME TABTYPE CLUSTERID
------------------------------ -------
----------
DEPT TABLE
EMP TABLE
BONUS TABLE
SALGRADE TABLE
T1 TABLE
STUDENT TABLE
RAMA TABLE
7 rows selected.
Insert some values into the
student table:
SQL> insert into student values('venkat');
1 row created.
SQL> commit;
Commit complete.
SQL> alter system checkpoint;
System altered.
Check the block number and
the datafile in which the data is stored.
SQL> select
dbms_rowid.rowid_relative_fno(rowid)
file_num,dbms_rowid.rowid_block_number(rowid) block_num, name from
student;
FILE_NUM BLOCK_NUM NAME
---------- ---------- --------------------
4 68 venkat
Check the data in the block
number 68 with the dd command:
[oracle@dg2 ~]$ dd bs=8k skip=68 count=1
if=/u02/oradata/DBACLASS/users01.dbf |strings
1+0 records in
1+0 records out
8192 bytes (8.2 kB) copied, 6.9861e-05
seconds, 117 MB/s
Insert another row, commit
the transaction and check the block and file information:
SQL> insert into student values('ram');
1 row created.
SQL> commit;
Commit complete.
SQL> select
dbms_rowid.rowid_relative_fno(rowid) "file num",
dbms_rowid.rowid_block_number(rowid) "block num",name from student;
file
num block num NAME
----------
---------- --------------------
4 68 venkat
4 68 ram
Make the database in Backup
mode:
SQL> alter tablespace users begin backup;
Tablespace altered.
Update the record in the
student table:
SQL> update student set
name='rama krishna' where name='ram';
1 row updated.
[oracle@dg2 ~]$ dd bs=8k
skip=68 count=1 if=/u02/oradata/DBACLASS/users01.dbf |strings
1+0 records in
1+0 records out
8192 bytes (8.2 kB) copied,
6.5554e-05 seconds, 125 MB/s
ram,
venkat
Run the checkpoint process
which creates an SCN and invoke the DBWn process to write the changed blocks to
the datafiles:
SQL> alter system
checkpoint;
System altered.
Now extract the information
in the 68 block of that particular data file. We can observe that the changes
made to the table (database) are written into the data file during the backup
mode.
[oracle@dg2 ~]$ dd bs=8k
skip=68 count=1 if=/u02/oradata/DBACLASS/users01.dbf |strings
1+0 records in
1+0 records out
8192 bytes (8.2 kB) copied,
7.0099e-05 seconds, 117 MB/s
rama krishna,
ram,
venkat
So, it is clear that the
data files can be writable during the hot backup mode.
End the tablespace backup
mode.
SQL> alter tablespace
users end backup;
Tablespace altered.
