UNCOMMITTED DATA IN DATAFILES

In this post, I will demonstrate various scenarios in single instance where uncommitted data gets written to datafiles.

The database writer can write uncommitted info also to datafiles as a result of following activities:

Flush buffer cache
Before a logfile gets overwritten
Manual checkpoint
To free buffers
Tablespace offline
Tablespace read only
Tablespace begin backup

– SETUP –

– create tablespace test_dbwr

SQL>drop tablespace test_dbwr including contents and datafiles;
    create tablespace test_dbwr datafile '/u01/app/oracle/test_dbwr.dbf' size 100m;

– create table test_dbwr_tab

SQL>drop table test_dbwr_tab purge;
    create table test_dbwr_tab (txt char(100)) tablespace test_dbwr;

- insert a record with txt = test_init

SQL>insert into test_dbwr_tab values ('test_init');
    commit;
    alter system checkpoint;

– find out current checkpoint#

SQL>select checkpoint_change# from v$database;

CHECKPOINT_CHANGE#
——————
983019

– Switch logfile

SQL>alter system switch logfile;

Flush buffer cache

If buffer cache containing dirty blocks is flushed, dirty blocks containing uncomitted data are written to datafiles.

— Overview–

– find out current checkpoint#
– Update record in test_dbwr_tab to ‘test_dbwr’ and do not commit
– check that datafile does not contain string ‘test_dbwr’ as checkpoint has not taken place yet
– Flush buffer cache so that all the dirty blocks in the buffer cache are
flushed to disk by database writer
– Check that checkpoint# has been incremented
– check that datafile contains the string ‘test_dbwr’ as dirty blocks
have been flushed to disk

Implementation

– find out current checkpoint#

SQL>select checkpoint_change# from v$database;

CHECKPOINT_CHANGE#
------------------
            983019

– Update record in test_dbwr_tab to ‘test_dbwr’ and do not commit

SQL>update test_dbwr_tab set txt = 'test_dbwr';

-- check that datafile does not contain string ‘test_dbwr’ as checkpoint has not taken place yet

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_dbwr

– Flush buffer cache so that all the dirty blocks in the buffer cache are flushed to disk by database writer

SQL>alter system flush buffer_cache;

– Check that checkpoint# has been incremented

SQL> select checkpoint_change# from v$database;

CHECKPOINT_CHANGE#
------------------
            983038

– check that datafile contains the string ‘test_dbwr’ as dirty blocks have been flushed to disk

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_dbwr
dtest_dbwr

————————————————————————

Before a logfile gets overwritten

A redo log cannot be overwritten until changes recorded in it are written to disk by DBWR. Hence, before a redo log containing uncommitted data is overwritten, uncommitted changes recorded in it are written to datafiles by DBWR.

– Overview –
– check the no. of logfile groups and the current logfile group
– Check current checkpoint#
– update record to string ‘test_log_switch’ in table test_dbwr_tab
– check that datafile does not contain string ‘test_log_switch’
as dirty buffers have not been written to disk yet
– Switch logfile so that curent redo log gets overwritten
– Check that checkpoint# has incremented
– check that datafile contains the string ‘test_log_switch’ as dirty buffers
have been flushed to disk

– Implementation –


— check the no. of logfile groups and the current logfile group

SQL>select group#, status from v$log ;
   GROUP# STATUS
---------- ----------------
         4 INACTIVE
         5 INACTIVE
         6 CURRENT

– Check current checkpoint#

SQL>select checkpoint_change# from V$database;

CHECKPOINT_CHANGE#
------------------
            983038

– update record to string ‘test_log_switch’ in table test_dbwr_tab

SQL>update test_dbwr_tab set txt = 'test_log_switch';

– check that datafile does not contain string ‘test_log_switch’  as dirty buffers have not been written to disk yet

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_log_switch

– Switch logfile so that curent redo log gets overwritten

SQL>alter system switch logfile;
    /
    /
    /
    select group#, status from v$log ;

– Check that checkpoint# has incremented

SQL>select checkpoint_change# from V$database;

CHECKPOINT_CHANGE#
------------------
            983337

– check that datafile contains the string ‘test_log_switch’ as dirty buffers
have been flushed to disk

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_log_switch
dtest_log_switch

————————————————————————–

Manual checkpoint

Whenever checkpoint is initiated manually, all the dirty blocks containing committed/uncommitted data in buffer cache are flushed to disk .

– Overview –
– Find out current checkpoint#
– Update the record to string ‘test_checkpoint’ in table test_dbwr_tab
– check that datafile does not contain string ‘test_checkpoint’
– Perform manual checkpoint so that all the dirty buffers get written
to disk
– Check that checkpoint# has incremented
– check that datafile contains the string ‘test_checkpoint’ as dirty buffers
have been flushed to disk

– Implementation –

– Find out current checkpoint#

SQL>select checkpoint_change# from V$database;

CHECKPOINT_CHANGE#
------------------
            983337

– Update the record to string ‘test_checkpoint’ in table test_dbwr_tab

SQL>update test_dbwr_tab set txt = 'test_checkpoint';

– check that datafile does not contain string ‘test_checkpoint’

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_checkpoint

– Perform manual checkpoint so that all the dirty buffers get written to disk

SQL>alter system  checkpoint;

– Check that checkpoint# has incremented

SQL>select checkpoint_change# from V$database;

CHECKPOINT_CHANGE#
------------------
            983842

– check that datafile contains the string ‘test_checkpoint’ as dirty buffers have been flushed to disk

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_checkpoint
dtest_checkpoint

——————————————————————

Tablespace offline

When a tablespace is taken offline, the  dirty  buffers containinig committed/uncommitted data belonging to it are flushed to disk.

– Overview –
– Find out current checkpoint#
– Find out current checkpoint# for datafile of test_dbwr tablespace
– Update the record to string ‘test_offline’ in table test_dbwr_tab
– check that datafile does not contain string ‘test_offline’
– Take tablespace test_dbwr offline
– Note that database checkpoint# does not change
– Note that checkpoint# for the datafile for test_dbwr tablespace has been incremented
– check that datafile contains the string ‘test_offline’ as dirty buffers
– have been flushed to disk
– Bring tablespace online –

– Implementation –

– Find out current checkpoint#

SQL>select checkpoint_change# from V$database;

CHECKPOINT_CHANGE#
------------------
            983842

– Find out current checkpoint# for datafile of test_dbwr tablespace

SQL>select checkpoint_change# from V$datafile
    where upper(name) like '%TEST_DBWR%';

CHECKPOINT_CHANGE#
------------------
            983842

– Update the record to string ‘test_offline’ in table test_dbwr_tab

SQL>update test_dbwr_tab set txt = 'test_offline';

– check that datafile does not contain string ‘test_offline’

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_offline

– Take tablespace test_dbwr offline –

SQL>alter tablespace test_dbwr offline;

– Note that database checkpoint# does not change

SQL>select checkpoint_change# from V$database;
CHECKPOINT_CHANGE#
------------------
            983842

– Note that checkpoint# for the datafile for test_dbwr tablespace has been incremented

SQL>select checkpoint_change# from V$datafile
    where upper(name) like '%TEST_DBWR%';

CHECKPOINT_CHANGE#
------------------
            984338

– check that datafile contains the string ‘test_offline’ as dirty buffers – have been flushed to disk

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_offline
dtest_offline

– Bring tablespace online –

SQL>alter tablespace test_dbwr online;

————————————————————————–

Similarly, it can be demonstrated that uncommitted data gets written to datafiles whenever
- a tablespace is put in read only mode
- a tablespace is put in backup mode

In these cases also, database checkpoint# is not incremented but checkpoint# for the datafile of the tablespace gets incremented

To free buffers in buffer cache

When a server process is looking for free buffers and they are not available, dirty buffers containing uncommitted/committed data from cold end of LRU list  will be flushed to disk. The flushed buffers may not be the ones with oldest SCN.  Hence, in this case checkpoint# is not incremented.

– Overview –

– Find out current size of database buffer cache
– Decrease size of buffer cache to small value 4M
– check that buffer cache has been shrunk to 4M
– check checkpoint# for the database –
– Update the record to string ‘test_buffer’ in table test_dbwr_tab
– Issue a query on a large table so that dirty buffers in buffer cache have to be flushed to disk in order to free buffers
– check that datafile now contain the strings ‘test_buffer’ as dirty buffers have been written to disk
– Note that checkpoint # has not changed in this case although dirty buffers have been written to disk(LRU/TCH algorithm)
– Reset buffer cache to earlier size

Implementation

– Find out current size of database buffer cache

SQL> col component for a20
     select component, current_size/1024/1024 current_mb, min_size/1024/1024 MIN_MB
     from v$sga_dynamic_components
     where component = 'DEFAULT buffer cache';

COMPONENT            CURRENT_MB     MIN_MB
-------------------- ---------- ----------
DEFAULT buffer cache         28         28

– Decrease size of buffer cache to minimum value i.e. 4M

SQL>alter system set db_cache_size=4m;

– check that buffer cache has been shrunk to 4M

SQL> select component, current_size/1024/1024 current_mb, min_size/1024/1024 MIN_MB
     from v$sga_dynamic_components
     where component = 'DEFAULT buffer cache';

OMPONENT            CURRENT_MB     MIN_MB
-------------------- ---------- ----------
DEFAULT buffer cache          4          4

– check checkpoint# for the database –

SQL>select checkpoint_change# from V$database;

CHECKPOINT_CHANGE#
------------------
            983842

– Update the record to string ‘test_buffer’ in table test_dbwr_tab

SQL>update test_dbwr_tab set txt = 'test_buffer';

– check that datafile does not contain string ‘test_buffer”

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_buffer

– Issue a query on a large table so that dirty buffers in buffer cache have to be flushed to disk in order to free buffers

SQL>select * from sh.sales;

– check that datafile now contain the strings ‘test_buffer’ as dirty buffers have been written to disk

[oracle@node1 oracle]$ strings /u01/app/oracle/test_dbwr.dbf| grep test_buffer
dtest_buffer

– Note that checkpoint # has not changed in this case although dirty buffers have been written to disk(LRU/TCH algorithm)

SQL>select checkpoint_change# from V$database;

CHECKPOINT_CHANGE#
------------------
            983842

– Reset buffer cache to earlier size

SQL>alter system set db_cache_size=44m;
    select component, current_size/1024/1024 current_mb, min_size/1024/1024 MIN_MB
     from v$sga_dynamic_components
     where component = 'DEFAULT buffer cache';

– cleanup –

SQL>drop tablespace test_dbwr including contents and datafiles;

Summary:

- When uncommitted data is flushed to disk as a result of

  •      . flush buffer cache,
  •      . manual checkpoint or
  •      . to write changes in a redo log to be overwritten,

checkpoint# for the database is incremented.

- When uncommitted data belonging to a tablespace  is flushed to disk as a result of

  •        . tablespace offline,
  •        . tablespace read only or
  •        . tablespace begin backup

checkpoint# for the database is not incremented but checkpoint# for the datafile(s) belonging to the tablespace is incremented.

- When uncommitted data   is flushed to disk to free buffers in the buffer cache,    checkpoint# for the database is not incremented .

References:

Oracle Documentation
———————————————————————–

Related links:

Home

Database  Index

Consistent Reads In Oracle : Part-I
Consistent Reads In Oracle : Part-II
Flush Buffer Cache
Oracle Checkpoints
Undo And Redo In Oracle

 

5 thoughts on “UNCOMMITTED DATA IN DATAFILES

  1. Thanks for update regarding DBWR and LGWR role in committed and uncommitted transaction in datafiles and wishing you good luck for your bright career in Oracle DBA.

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