switch logfile

不少Oracle的初学者都会因为checkpoint这个知识点而头痛，绝大多数Oracle文档对完全检查点和增量检查点的描述又都略显朦胧；譬如在线日志的切换引起的是完全检查点还是增量检查点这个问题，就有不少的争论。实际上增量检查点与完全检查点有一个显著的区别:完全检查点发生时控制文件和数据文件头中的checkpoint scn都会被更新，而增量检查点发生时只有控制文件中的checkpoint scn更新;
我们可以通过以下演示证明日志切换引发的到底是何种检查点?:

SQL> select checkpoint_change# from v$datafile_header where status='ONLINE';
CHECKPOINT_CHANGE#
------------------
           1665476
           1665476
           1665476
           1665476
           1665476
           1665476
6 rows selected.

SQL> alter system checkpoint;
System altered.

SQL> select checkpoint_change# from v$datafile_header where status='ONLINE';
CHECKPOINT_CHANGE#

------------------
           1697131
           1697131
           1697131
           1697131
           1697131
           1697131
6 rows selected.

/* 手动执行checkpoint,数据文件头的checkpoint scn立即更新了 */

SQL> alter system flush buffer_cache;
System altered.

SQL> select checkpoint_change# from v$datafile_header where status='ONLINE';
CHECKPOINT_CHANGE#
------------------
           1697131
           1697131
           1697131
           1697131
           1697131
           1697131
6 rows selected.

/* 单纯flush buffer cache冲刷数据库高速缓存不会更新数据文件头的checkpoint scn */

SQL> alter system set log_checkpoints_to_alert=true;
System altered.

SQL> alter system set log_checkpoint_timeout=20;
System altered.

/* 设置log_checkpoint_timeout为20s，频繁引发增量检查点 */

alert log:
Wed Nov  3 20:24:49 2010
Incremental checkpoint up to RBA [0x3d.dff1.0], current log tail at RBA [0x3d.dff6.0]
Wed Nov  3 20:25:07 2010
Incremental checkpoint up to RBA [0x3d.dff7.0], current log tail at RBA [0x3d.dffc.0]
Wed Nov  3 20:25:25 2010
Incremental checkpoint up to RBA [0x3d.dffd.0], current log tail at RBA [0x3d.e002.0]
Wed Nov  3 20:25:43 2010
Incremental checkpoint up to RBA [0x3d.e003.0], current log tail at RBA [0x3d.e008.0]
Wed Nov  3 20:26:01 2010
Incremental checkpoint up to RBA [0x3d.e009.0], current log tail at RBA [0x3d.e00e.0]

SQL> set time on;

20:26:38 SQL> select checkpoint_change# from v$datafile_header where status='ONLINE';
CHECKPOINT_CHANGE#
------------------
           1697131
           1697131
           1697131
           1697131
           1697131
           1697131

6 rows selected.

/* 可以看到增量检查点并不会引起数据文件头的checkpoint scn 被更新 */

20:26:43 SQL>  alter system set log_checkpoint_timeout=1800;
System altered.

/* 那么日志文件切换就会引起数据文件头的checkpoint scn被更新吗？*/

20:28:10 SQL> alter system switch logfile;
System altered.

20:29:16 SQL> select checkpoint_change# from v$datafile_header where status='ONLINE';
CHECKPOINT_CHANGE#
------------------
           1697131
           1697131
           1697131
           1697131
           1697131
           1697131
6 rows selected.

/* logfile switch 日志文件切换引起的是一种slow慢的完全检查点，它不同于alter system checkpoint(ASC)，
   ASC要求的脏块写出和控制文件及数据文件头更新时要立即完成的，也就是说当alter system checkpoint语句返回"System altered."
   后以上工作都已经完成了；而alter system switch logfile或者自然的日志切换引发的是一种慢的完全检查点，
   它在返回"System altered"时不要求写脏块等工作必须已经完成
*/

/* 我们可以用冲刷高速缓存的方式保证脏块写出的工作被督促完成 */

20:33:39 SQL> alter system flush buffer_cache;
System altered.

20:33:45 SQL> select checkpoint_change# from v$datafile_header where status='ONLINE';
CHECKPOINT_CHANGE#
------------------
           1697544
           1697544
           1697544
           1697544
           1697544
           1697544

6 rows selected.

/* 虽然日志切换所引发的slow checkpoint(慢的检查点)并无立即完成的要求，但也并非全无限制；
   当某次日志切换由1号日志组切换到2号日志组时，
   将引发一个slow checkpoint，之后日志连续切换又要切到1号日志组时要求之前的那个slow checkpoint在切换前必须完成
*/

20:41:35 SQL> set timing on;

20:42:02 SQL>  select * from v$log;
    GROUP#    THREAD#  SEQUENCE#      BYTES    MEMBERS ARC STATUS           FIRST_CHANGE# FIRST_TIME
---------- ---------- ---------- ---------- ---------- --- ---------------- ------------- -------------------
         1          1         67   52428800          2 YES INACTIVE               1698288 2010-11-03 20:41:19
         2          1         68   52428800          2 YES INACTIVE               1698292 2010-11-03 20:41:21
         3          1         69   52428800          2 NO  CURRENT                1698302 2010-11-03 20:41:35
Elapsed: 00:00:00.00

20:42:17 SQL> delete tv;
51134 rows deleted.
Elapsed: 00:00:01.68

20:42:34 SQL> commit;
Commit complete.
Elapsed: 00:00:00.00

20:42:36 SQL> alter system switch logfile;
System altered.
Elapsed: 00:00:00.01

20:42:40 SQL> alter system switch logfile;
System altered.
Elapsed: 00:00:00.01

20:42:43 SQL> alter system switch logfile;
System altered.
Elapsed: 00:00:02.00

20:45:28 SQL>  select checkpoint_change# from v$datafile_header where status='ONLINE';
CHECKPOINT_CHANGE#
------------------
           1700686
           1700686
           1700686
           1700686
           1700686
           1700686
6 rows selected.
Elapsed: 00:00:00.00

alter.log告警日志中的内容:

Wed Nov  3 20:42:40 2010
Beginning log switch checkpoint up to RBA [0x46.2.10], SCN: 1700686
...........................
Wed Nov  3 20:42:45 2010
Thread 1 cannot allocate new log, sequence 72
Checkpoint not complete
....................
Completed checkpoint up to RBA [0x46.2.10], SCN: 1700686

/* 最近一次的日志切换耗费2s，在告警日志中可以看到此次slow checkpoint的相关记录 */

logfile switch causes incremental checkpoint?