- checkpoint 会导致脏数据写入datafile；

- buffers 会被写入当前的undo 表空间

SQL> show parameter undo_tablespace


NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_tablespace                      string      UNDOTBS1

SQL> create undo tablespace undotbs2 datafile ‘/u01/app/oracle/undotbs2.dbf‘
  2  size 100m;

Tablespace created.

SQL> create tablespace test_undo datafile ‘/u01/app/oracle/test_undo.dbf‘
  2  size 128k;

Tablespace created.

SQL> create table test_undo_tab(txt char(1000)) tablespace test_undo;

Table created.

SQL> insert into test_undo_tab values (‘teststring1‘);

1 row created.

SQL> insert into test_undo_tab values (‘teststring2‘);

1 row created.

SQL> commit;

SQL> alter system checkpoint;

System altered.

SQL> alter system set undo_tablespace=undotbs2;

System altered.

SQL> show parameter undo_tablespace;

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
undo_tablespace                      string      UNDOTBS2

SQL> alter system switch logfile;


System altered.

SQL> col member for a30
SQL> select member, l.status from v$log l, v$logfile f
  2  where l.group# = f.group#
  3  and l.status = ‘CURRENT‘;

MEMBER                         STATUS
------------------------------ ----------------
/u01/app/oracle/oradata/orcl/r CURRENT
edo02.log

SQL> update test_undo_tab set txt = ‘teststring_uncommitted‘
  2  where txt = ‘teststring1‘;

1 row updated.

SQL>  update test_undo_tab set txt = ‘teststring_committed‘
      where txt = ‘teststring2‘;
      commit;

[oracle@dylan ~]$  strings /u01/app/oracle/oradata/orcl/redo02.log | grep teststring

teststring_uncommitted                                                                                  
teststring1                                                          

teststring_committed                                                 

teststring2

[oracle@dylan ~]$ strings /u01/app/oracle/test_undo.dbf | grep teststring

teststring2                                                                  
teststring1

SQL> alter system checkpoint;

[oracle@dylan ~$ strings /u01/app/oracle/test_undo.dbf|grep teststring

teststring_committed                                                                                                               ,
teststring_uncommitted

[oracle@dylan ~]$ strings /u01/app/oracle/undotbs2.dbf | grep teststring

teststring2                                                                  
teststring1

SQL>drop tablespace test_undo including contents and datafiles;
    alter system set undo_tablespace=undotbs1;
    drop tablespace undotbs2 including contents and datafiles;

Let’s see what will happen if undo is stored in redo logs only.

假设仅将undo信息存储于redo logs会怎么样？

A redo log can be reused once changes protected by it have been written to datafiles (and archivelogs if database is in archivelog mode).

It implies that if I make a change and do not commit it 
- Change is written to a redo log  假设我改变的数据而没提交。此时改变将记录到redo log
- checkpoint takes place  检查点发生
- uncommitted change is written to datafile  后未提交的数据写入了数据文件
- I decide to rollback the change  这时我打算回滚
- If redo log has not been overwritten  假设redo log没被覆盖
. search entire redo log for the undo and then rollback  那么搜素整个redo log进行回滚操作
else (redo log has been overwritten)
. undo information is not available for rollback.    否则将无法回滚，undo信息已丢失！

One might argue that if somehow a redo log is not allowed to be overwritten until it contains active undo, we might be able to manage with undo stored in redo logs only. This solution is not feasible as
- size of redo logs will grow enormously large very soon as thet contain both undo and redo (a user might decide not to end a transaction for months)
- to rollback a change, enormous amount of data in redo logs (both redo and undo) will have to be searched leading to degraded performance
- there will be contention on redo logs as they are being used for both
. writing redo and undo
. reading to rollback a change 

有人或许会争论：那就不同意redo log 覆盖undo 信息直到包括新的undo，这样redo log将变得异常大从而影响系统性能！

Hence, undo information has to be stored separately from redo and is used for rolling back uncommited transactions . The undo stored in undo buffers/undo tablespace is additionally used for
- read consistency   读一致性
- flashback query      闪回查询
- flashback version query   闪回版本号查询