困惑中，logfile switch产生的checkpoint到底是增量的还是完全的呢？

logswitch

无聊中研究checkpoint，困惑于logfile switch到底产生那种类型的checkpoint，google+asktom+metalink+咨询数位大佬后发现有下面几种意见，但是都缺少有说服力的文档说明
1、绝对就是普通的增量checkpoint，完全checkpoint怎么会要那么长的时间呢
2、是增量checkpoint但是是会更新scn信息的那种
3、当然是完全的啦，只不过是属于优先级比较低的那种，不会推动dbwr立即写数据文件，所以完成时间较长



不知道坛中各位大佬是什么意见，都支持那种说法呢？希望各位在说出自己意见同时给出强有力的证明。

zhouwf0726

The incremental checkpoint technique uses the same data
structures that are used by conventional checkpoints. It
exploits the fact that the dirty buffers in the cache are
linked in low RBA order. If DBWR continually writes
buffers from the head of the checkpoint queue, the instance
checkpoint (lowest low-RBA of the modified buffers) will
keep advancing. Periodically, CKPT can record this lowest
low-RBA to the control file (using a very lightweight control-
file update protocol). This periodically recorded lowest
low RBA is the current position of the incremental
checkpoint for the instance. Since the incremental checkpoint
is performed continuously, the value of the incremental
checkpoint RBA will be much closer to the tail of
the log than the RBA of a “conventional” checkpoint, thus
limiting the amount of recovery needed. When incremental
checkpointing is enabled, DBWR keeps writing buffers
from the checkpoint queues in ascending low RBA order
in addition to performing other writing activity. In addition,
the CKPT process periodically records the progress
of the incremental checkpoint in the control file. By controlling
the rate at which buffers are written, we can reduce
the overhead for incremental checkpoint. Quite often,
writing buffers in ascending low RBA order also performs
LRU replacement writes and vice versa. Hence, aging
writes and checkpoint writes can complement each other.

logswitch

感谢周版主的回复
-----------

下面这些操作将会触发checkpoint事件：
日志切换，
通过ALTER SYSTEM SWITCH LOGFILE。
DBA发出checkpoint命令，通过ALTER SYSTEM checkpoint。
对数据文件进行热备时，针对该数据文件的checkpoint也会进行，ALTER TABLESPACE TS_NAME BEGIN BACKUP/END BACKUP。
当运行ALTER TABLESPACE/DATAFILE READ ONLY的时候。
SHUTDOWN命令发出时。  

完全检查点：
定义：清除脏列表(DIRTY LIST OR CHECKPOINT ENQUEUE)中所有数据块。
什么时候发生：ALTER SYSTEM CHECKPOINT; SHUTDOWN;  

--------------

这是其中的一个观点，说完全checkpoint只在上面说的两种情况下发生，没有logfile switch。

logswitch

日志切换时的检查点

-- 我们先做一次日志切换
ALTER SYSTEM SWITCH LOGFILE;

-- 然后看看alert里面的记录
Mon Aug  4 22:31:39 2008
Beginning log switch checkpoint up to RBA [0x9.2.10], SCN: 534450
Thread 1 advanced to log sequence 9
  Current log# 2 seq# 9 mem# 0: /u/app/oracle/oradata/orcl/redo02.log
Mon Aug  4 22:35:58 2008
Completed checkpoint up to RBA [0x9.2.10], SCN: 534450

-- 我们能看到checkpoint是在过了一段时间（这里是4分钟）之后才完成的

-- 接着我们来看下V$DATAFILE_HEADER中的结果
NO  STATUS  TABLESPACE_NAME  CUR_SCN  RST_DT              RST_SCN  CKPT_DT             CKPT_SCN  CKPT_CNT
--- ------- ---------------- -------- ------------------- -------- ------------------- --------- ---------
1   ONLINE  SYSTEM           534770   2008-01-12 16:51:53 446075   2008-08-04 22:31:44 534450    67
2   ONLINE  UNDOTBS1         534770   2008-01-12 16:51:53 446075   2008-08-04 22:31:44 534450    30
3   ONLINE  SYSAUX           534770   2008-01-12 16:51:53 446075   2008-08-04 22:31:44 534450    67
4   ONLINE  USERS            534770   2008-01-12 16:51:53 446075   2008-08-04 22:31:44 534450    66
5   ONLINE  EXAMPLE          534770   2008-01-12 16:51:53 446075   2008-08-04 22:31:44 534450    26

-- 在这里我们能发现下V$DATAFILE_HEADER里面记录的SCN和日志切换发生的checkpoint的SCN是一样的，
-- 这就证明了日志切换是会更新数据文件头的，同时日志切换的checkpoint是一个级别比较低的操作，
-- 它不会立即完成，这也是出于性能上考虑的。

oracledba

logswitch

checkpoint_instructors.ppt里面的观点是这个（P26）

CKPT also writes the checkpoint position to the datafile headers but only after a log switch.  
Note: log switch does not cause a full checkpoint.

langzhi000

增量 CHECKPOINT;

paulyibinyi

增量 CHECKPOINT

oracledba

别光说个增量，解释一下为什么

Sea_jk

顶，看来也需要再对check_point了解一下了。