如果LGWR的下一个日志是ACTIVE，那么LWGR会挂起，警告日志会报告"Checkpoint not complete",oracle会发起alter system checkpoint的操作 状态为ACTIVE的日志也可能已经是归档日志了，ARCn进程会自动将非CURRENT的在线日志归档
V$LOG.STATUS反应的是完全检查点的进度，因为alter system switch logfile后还是会发现原来为ACTIVE的日志还是ACTIVE状态，但是alter system checkpoint后就一定会把ACTIVE变成INACTIVE(完全检查点写入控制文件和数据文件头部，增量检查点只写入控制文件) V$LOG.STATUS=ACTIVE Log is active but is not the current log. It is needed for crash recovery. It may be in use for block recovery. It may or may not be archived. 代表最近一次的完全检查点SCN小于该日志中最后一条重做记录的SCN,说明完全检查点还没有越过这个在线日志 V$LOG.STATUS=INACTIVE Log is no longer needed for instance recovery. It may be in use for media recovery. It may or may not be archived. 代表最近一次的完全检查点SCN大于该日志中最后一条重做记录的SCN,说明完全检查点已经越过这个在线日志
v$log.FIRST_CHANGE#:等于上一个online redo的v$log.NEXT_CHANGE#或上一个archive redo log的$archived_log.NEXT_CHANGE#，等于下一个archive redo log的v$archived_log.FIRST_CHANGE# v$log.NEXT_CHANGE#:Highest change number (SCN) in the log. When STATUS=CURRENT, NEXT_CHANGE# is set to the highest possible SCN, 281474976710655
V$DATABASE displays information about the database from the control file. V$DATABASE.CHECKPOINT_CHANGE#：Last SCN checkpointed V$DATABASE.CONTROLFILE_CHANGE#：Last SCN in backup control file; null if the control file is not a backup V$DATABASE.CURRENT_SCN：Current SCN; null if the database is not currently open. For a standby database, it is the checkpoint SCN of the mounted physical standby database during media recovery and is always less than the last applied SCN tracked in V$RECOVERY_PROGRESS. V$DATAFILE displays datafile information from the control file. V$DATAFILE.CHECKPOINT_CHANGE#：SCN at last checkpoint
V$DATAFILE_HEADER displays datafile information from the datafile headers. V$DATAFILE_HEADER.CHECKPOINT_CHANGE#：Datafile checkpoint change#
执行alter system switch logfile时，只有V$DATABASE.CONTROLFILE_CHANGE#变大了，其他V$DATABASE.CHECKPOINT_CHANGE#、V$DATAFILE.CHECKPOINT_CHANGE#、V$DATAFILE_HEADER.CHECKPOINT_CHANGE#都没有变
执行ALTER DATABASE BEGIN BACKUP时，V$DATABASE.CONTROLFILE_CHANGE#、V$DATAFILE.CHECKPOINT_CHANGE#、V$DATAFILE_HEADER.CHECKPOINT_CHANGE#都变大了，只有V$DATABASE.CHECKPOINT_CHANGE#没变
执行alter tablespace users begin backup时，V$DATABASE.CONTROLFILE_CHANGE#、对应的表空间文件V$DATAFILE.CHECKPOINT_CHANGE#、对应的表空间文件V$DATAFILE_HEADER.CHECKPOINT_CHANGE#变大了，$DATABASE.CHECKPOINT_CHANGE#没变，其他表空间文件的V$DATAFILE.CHECKPOINT_CHANGE#、$DATAFILE_HEADER.CHECKPOINT_CHANGE#也没变
执行alter system checkpoint时，则V$DATABASE.CONTROLFILE_CHANGE#、V$DATABASE.CHECKPOINT_CHANGE#、V$DATAFILE.CHECKPOINT_CHANGE#、V$DATAFILE_HEADER.CHECKPOINT_CHANGE#都变大了
查询相应SCN的一些语句 select CHECKPOINT_CHANGE#,CONTROLFILE_CHANGE#,CURRENT_SCN from v$database; select distinct CHECKPOINT_CHANGE#,last_change# from v$datafile; select distinct CHECKPOINT_CHANGE# from v$datafile_header; select distinct FIRST_CHANGE#,NEXT_CHANGE#,sequence#,first_time from v$archived_log order by first_time desc; select SEQUENCE#,STATUS,ARCHIVED,FIRST_CHANGE#,NEXT_CHANGE# from v$log
mount状态下scn不会改变
查询当前SCN的SQL 一般使用如下 select to_char(dbms_flashback.get_system_change_number) scn from dual mount状态执行上述语句会报错ORA-00904
下面这条查询语句，每执行一次scn就会增加一次 select CURRENT_SCN from v$database; mount状态下执行上述语句结果是0
COMMIT Statement The COMMIT statement ends the current transaction, making its changes permanent and visible to other users. COMMIT不会触发任何的checkpoint，只是触发lgwr把日志缓冲数据写入在线重做日志并把事务对应的数据块的最新scn和是否的提交状态记录在控制文件中，但是不会记录在任何视图中，V$DATABASE.CURRENT_SCN虽然也来自来自控制文件，但是记录当前最新的SCN
database checkpoint The thread checkpoint that has the lowest SCN. All changes in all enabled redo threads with SCNs before the database checkpoint SCN are guaranteed to have been written to disk. 具有最低SCN的线程检查点。 在数据库检查点SCN之前所有启用的具有SCN的重做线程的所有更改都保证已写入磁盘。
data file checkpoint A data structure that defines an SCN in the redo thread of a database for a particular data file. Every data file has a checkpoint SCN, which you can view in V$DATAFILE.CHECKPOINT_CHANGE#. All changes with an SCN lower than this SCN are guaranteed to be in the data file. 数据结构，用于定义特定数据文件的数据库重做线程中的SCN。 每个数据文件都有一个检查点SCN，您可以在V$DATAFILE.CHECKPOINT_CHANGE＃中查看。 SCN低于此SCN的所有更改都将保证在数据文件中。
Overview of Checkpoints A checkpoint is a crucial mechanism in consistent database shutdowns, instance recovery, and Oracle Database operation generally. The term checkpoint has the following related meanings: A data structure that indicates the checkpoint position, which is the SCN in the redo stream where instance recovery must begin The checkpoint position is determined by the oldest dirty buffer in the database buffer cache. The checkpoint position acts as a pointer to the redo stream and is stored in the control file and in each data file header. The writing of modified database buffers in the database buffer cache to disk 检查点是一致的数据库关闭，实例恢复和Oracle数据库操作的关键机制。 检查点一词具有以下相关含义： 指示检查点位置的数据结构，该位置是实例恢复必须开始的重做流中的SCN 检查点位置由数据库缓冲区缓存中最旧的脏缓冲区确定。 检查点位置用作指向重做流的指针，存储在控制文件和每个数据文件头中。 将数据库缓冲区中的修改后的数据库缓冲区写入磁盘
When Oracle Database Initiates Checkpoints The checkpoint process (CKPT) is responsible for writing checkpoints to the data file headers and control file. Checkpoints occur in a variety of situations. For example, Oracle Database uses the following types of checkpoints: Thread checkpoints The database writes to disk all buffers modified by redo in a specific thread before a certain target. The set of thread checkpoints on all instances in a database is a database checkpoint. Thread checkpoints occur in the following situations: Consistent database shutdown ALTER SYSTEM CHECKPOINT statement Online redo log switch ALTER DATABASE BEGIN BACKUP statement Tablespace and data file checkpoints The database writes to disk all buffers modified by redo before a specific target. A tablespace checkpoint is a set of data file checkpoints, one for each data file in the tablespace. These checkpoints occur in a variety of situations, including making a tablespace read-only or taking it offline normal, shrinking a data file, or executing ALTER TABLESPACE BEGIN BACKUP. 检查点进程（CKPT）负责将检查点写入数据文件头文件和控制文件。 检查点发生在各种情况。 例如，Oracle数据库使用以下类型的检查点： 线程检查点 数据库通过在某个目标之前的特定线程中重做修改的所有缓冲区写入磁盘。 数据库中所有实例上的一组线程检查点是一个数据库检查点。 线程检查点在以下情况下发生： 一致的数据库关机 ALTER SYSTEM CHECKPOINT语句 在线重做日志切换 ALTER DATABASE BEGIN BACKUP语句 表空间和数据文件检查点 数据库将磁盘上的所有缓冲区写入特定目标之前通过重做修改。 表空间检查点是一组数据文件检查点，一个用于表空间中的每个数据文件。 这些检查点发生在各种情况下，包括使表空间为只读或正常脱机，收缩数据文件或执行ALTER TABLESPACE BEGIN BACKUP。
Incremental checkpoints An incremental checkpoint is a type of thread checkpoint partly intended to avoid writing large numbers of blocks at online redo log switches. DBWn checks at least every three seconds to determine whether it has work to do. When DBWn writes dirty buffers, it advances the checkpoint position, causing CKPT to write the checkpoint position to the control file, but not to the data file headers. Other types of checkpoints include instance and media recovery checkpoints and checkpoints when schema objects are dropped or truncated. 增量检查点是一种类型的线程检查点，部分是为了避免在线重做日志切换中写入大量块。 DBWn至少每三秒检查一次，以确定是否有工作要做。 当DBWn写入脏缓冲区时，它会提前检查点位置，导致CKPT将检查点位置写入控制文件，而不是数据文件头。 其他类型的检查点在模式对象被删除或截断时包括实例和介质恢复检查点和检查点。
增量检查点 增量检查点会推动dbwr将部分脏数据块写回数据文件，但是检查点SCN只是记录到控制文件即V$DATABASE.CONTROLFILE_CHANGE#，而没有写入数据文件头部。实例恢复操作并不会因此而少索取日志记录。比如：增量检查点SCN到200了，意味着数据文件内数据块的最高的SCN也已经是200了，但是数据文件头部可能还写着检查点SCN是100,如果此时实例崩溃，实例恢复的前滚会从scn为100的重做记录开始，实际上从scn号100~200的重做记录根本不需要前滚。oracle当然了解这一点，所以每次写完脏数据块之后，dbwr会添加一条被称为BWR（block written record，数据块已写）的重做记录，该记录的变更矢量不代表任何变更，只是用来标记哪些数据块已经被写回数据文件了。 因为有BWR这样的记录，oracle在进行自动前滚时实际上采用“两次读取法”读取在线日志，仍然假设数据文件检查点scn为100，增量检查点已经到200了，第一次读取日志中scn号100后的所有重做记录，目的是确定所有的真正的需要恢复的数据块有哪些，BWR记录就像一个过滤器，可以大量减少这样的数据块。第二次读取在线日志时oracle只对真正需要恢复的数据块，即没有被BWR记录点到名的数据块的重做记录感兴趣。结果是oracle会跳过scn从100到200之间的重做记录，也就会使前滚相应变快
增量检查点作用： 1.减少发生完全检查点是dbwr进程的工作负担 2.提高实例恢复的速度
增量检查点发生时机 1.oracle自动控制(当三个参数都不设置或三个参数都设置不当时FAST_START_MTTR_TARGET、LOG_CHECKPOINT_TIMEOUT、LOG_CHECKPOINT_INTERVAL) 2.三个参数取最严厉的那个(FAST_START_MTTR_TARGET、LOG_CHECKPOINT_TIMEOUT、LOG_CHECKPOINT_INTERVAL) 3.lgwr切换在线日志 --alter system flush buffer_cache不会引发增加检查点
FAST_START_MTTR_TARGET enables you to specify the number of seconds the database takes to perform crash recovery of a single instance. When specified,FAST_START_MTTR_TARGET is overridden by LOG_CHECKPOINT_INTERVAL. 当设置了LOG_CHECKPOINT_INTERVAL时，LOG_CHECKPOINT_INTERVAL的设置会覆盖FAST_START_MTTR_TARGET的设置，而不是说11G已经取消了FAST_START_MTTR_TARGET这个参数的功能。
LOG_CHECKPOINT_INTERVAL specifies the frequency of checkpoints in terms of the number of redo log file blocks that can exist between an incremental checkpoint and the last block written to the redo log. This number refers to physical operating system blocks, not database blocks 这里指的blocks是OS的block，而不是DATABASE的block Regardless of this value, a checkpoint always occurs when switching from one online redo log file to another. Therefore, if the value exceeds the actual redo log file size,checkpoints occur only when switching logs. Checkpoint frequency is one of the factors that influence the time required for the database to recover from an unexpected failure 当LOG_CHECKPOINT_INTERVAL值大于redo log file size时，增量检查点发生情况就是在线日志切换取代LOG_CHECKPOINT_INTERVAL Specifying a value of 0 (zero) for LOG_CHECKPOINT_INTERVAL has the same effect as setting the parameter to infinity and causes the parameter to be ignored. Only nonzero values of this parameter are considered meaningful. 当LOG_CHECKPOINT_INTERVAL为0时，LOG_CHECKPOINT_INTERVAL这个参数就不起作用了 Recovery I/O can also be limited by setting the LOG_CHECKPOINT_TIMEOUT parameter or by the size specified for the smallest redo log. LOG_CHECKPOINT_TIMEOUT和LOG_CHECKPOINT_INTERVAL都生效，但是取两者更严厉的那个
LOG_CHECKPOINT_TIMEOUT specifies (in seconds) the amount of time that has passed since the incremental checkpoint at the position where the last write to the redo log(sometimes called the tail of the log) occurred. This parameter also signifies that no buffer will remain dirty (in the cache) for more than integer seconds. Specifying a value of 0 for the timeout disables time-based checkpoints. Hence, setting the value to 0 is not recommended unless FAST_START_MTTR_TARGET is set 不建议设置LOG_CHECKPOINT_TIMEOUT为0，除非你设置了FAST_START_MTTR_TARGET FAST_START_MTTR_TARGET、LOG_CHECKPOINT_INTERVAL为0时，LOG_CHECKPOINT_TIMEOUT也生效的
RAMN经常遇到的1号文件system太新的问题 如下三者都是说file 1太新了，file 1需要更多的恢复 ORA-01194: file 1 needs more recovery to be consistent ORA-01113: file 1 needs media recovery RMAN-06556: datafile 1 must be restored from backup older than scn 919248820 datafile 1的scn大于919248820，也就是datafile 1太新了，不够旧不够老 比如正常关机后，startup mount状态， sql直接recover database会报错ORA-00264: no recovery required rman直接recover database until sequence到前面几个archivelog就会报RMAN-06556
