很多人都以为Checkpoint只是将已经完成Transcation的脏页写入磁盘,其实不是这样的:
One commonmisconception about checkpoints is that they only write out pages with changesfrom committed transactions. This is not true—a checkpoint always writes outall dirty pages, regardless of whether the transaction that changed a page hascommitted or
not.(来自:http://technet.microsoft.com/en-us/magazine/2009.02.logging.aspx)
这里开启Traceflag 3503做个测试:
CREATEDATABASECheckpointTest;
GO
USECheckpointTest;
GO
CREATE TABLE t1(c1
INT IDENTITY, c2
CHAR (8000)
DEFAULT 'a');
go
CREATE CLUSTERED
INDEX t1c1 on t1(c1);
GO
SET NOCOUNT
ON;
GO
CHECKPOINT;
GO
DBCC TRACEON(3505,
-1);
GO
BEGIN TRAN;
INSERT INTO t1
DEFAULT VALUES;
GO
CHECKPOINT;
Errorlog中看到的结果:
2012-07-2610:14:29.180 spid57 Ckpt dbid 7 started (8)
2012-07-26 10:14:29.180 spid57 About to log Checkpoint begin.
2012-07-26 10:14:29.190 spid57 Ckpt dbid 7 phase 1 ended (8)
2012-07-26 10:14:29.190 spid57 FlushCache: cleaned up 6 bufs with 6 writes in 3ms (avoided 0 new dirty bufs)
2012-07-26 10:14:29.190 spid57 average throughput: 15.63 MB/sec, I/Osaturation: 5, context switches 6
2012-07-26 10:14:29.190 spid57 last target outstanding: 2, avgWriteLatency 0
2012-07-26 10:14:29.190 spid57 About to log Checkpoint end.
2012-07-26 10:14:29.190 spid57 Ckpt dbid 7 complete
可以看到虽然TRAN没有Commit但是脏页已经被写到磁盘了。
Trace flag3505请参考:http://support.microsoft.com/kb/815436
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无论事务是否已经提交,其所影响的页都会在Checkpoint时写回磁盘。但对于TempDB来说例外,因为TempDB的Checkpoint的事件周期中并不包含将脏页写回磁盘的步骤。 如果你想了解更多,请阅读下面文章: Technet 杂志文章...
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