如果设置了guarantee，意味着所有undo在undo retention指定的时间范围内都不能被覆盖，如果在此时间段内有大量的小事务，虽然每个事务使用的undo不大，但总量是巨大的，那么undo tablespace 将很快的被耗光，具体测试如下：

SQL> create undo tablespace undotbs1 datafile '/ora10g/oradata/ebank/undotbs1_01.dbf'
size 100m retention guarantee;

Tablespace created.

SQL> alter system set undo_tablespace=undotbs1 scope=both;

System altered.

SQL> drop tablespace undotbs including contents and datafiles;

Tablespace dropped.

SQL> select status ,sum(bytes)/1024/1024 from dba_undo_extents group by status;

STATUS    SUM(BYTES)/1024/1024
--------- --------------------
EXPIRED                   1.25

SQL> conn scott/tiger
Connected.
SQL> set timing on
SQL>
SQL> begin
 2  for i in 1 .. 100000 loop
 3  delete from test where rownum<=10;
 4  insert into test select * from emp where rownum<=10;
 5  commit;
 6  end loop;
 7  end;
 8  /
begin
*
ERROR at line 1:
ORA-30036: unable to extend segment by 8 in undo tablespace 'UNDOTBS1'
ORA-06512: at line 3

Elapsed: 00:01:17.83
SQL> conn /as sysdba
Connected.
SQL>  select status ,sum(bytes)/1024/1024 from dba_undo_extents group by status;

STATUS    SUM(BYTES)/1024/1024
--------- --------------------
UNEXPIRED               95.875
EXPIRED                      4

Elapsed: 00:00:00.09

因此，对于guarantee的设置一定要慎重。