Checkpoint原理浅析（12页）.pdf

1、Checkpoint原理浅析杨向博 DBACheckpoint 定义Checkpoint 触发条件Checkpoint 会做什么Checkpoint skipped机制Checkpoint 过程记录Checkpoint 定义Checkpointsarepointsinthesequenceoftransactionsatwhichitisguaranteedthattheheapandindexdatafileshavebeenupdatedwithallinformationwrittenbeforethatcheckpoint.Atcheckpointtime,alldirtydatapa

2、gesareflushedtodiskandaspecialcheckpointrecordiswrittentothelogfile.(ThechangerecordswerepreviouslyflushedtotheWALfiles.)Intheeventofacrash,thecrashrecoveryprocedurelooksatthelatestcheckpointrecordtodeterminethepointinthelog(knownastheredorecord)fromwhichitshouldstarttheREDOoperation.Anychangesmadet

3、odatafilesbeforethatpointareguaranteedtobealreadyondisk.Hence,afteracheckpoint,logsegmentsprecedingtheonecontainingtheredorecordarenolongerneededandcanberecycledorremoved.(WhenWALarchivingisbeingdone,thelogsegmentsmustbearchivedbeforebeingrecycledorremoved.)简单来说，checkpoint就是一个事务顺序的记录点。checkpoint主要是进

4、行刷脏页，redo时会参考checkpoint进行日志回放。除了刷脏之外还会更新一些位点信息，清理一些不再需要的wal。Checkpoint 触发条件在PostgreSQL中Checkpoint是由checkpointer进程执行的。Checkpointer进程的主流程是一个无条件的for循环，在未触发checkpoint时一直在WaitLatch中sleep，也就是在epoll_wait中观察list链表，查看是否有事件句柄已经就绪（某个条件在触发checkpoint）；如果已经存在就绪事件，则wakeup（通过SetLatch中writepipe的方式wakeup），执行checkpoin

5、t。Checkpoint 触发条件触发checkpoint的Flags:/*ThesedirectlyaffectthebehaviorofCreateCheckPointandsubsidiaries*/#defineCHECKPOINT_IS_SHUTDOWN0 x0001/*Checkpointisforshutdown*/#defineCHECKPOINT_END_OF_RECOVERY.0 x0002/*Likeshutdowncheckpoint,but*issuedatendofWALrecovery*/#defineCHECKPOINT_IMMEDIATE0 x0004/*Do

6、itwithoutdelays*/#defineCHECKPOINT_FORCE0 x0008/*Forceevenifnoactivity*/#defineCHECKPOINT_FLUSH_ALL0 x0010/*Flushallpages,includingthose*belongingtounloggedtables*/*TheseareimportanttoRequestCheckpoint*/#defineCHECKPOINT_WAIT0 x0020/*Waitforcompletion*/#defineCHECKPOINT_REQUESTED0 x0040/*Checkpointr