| Checkpoint & SCN [message #159122] |
Thu, 16 February 2006 05:18  |
gajini
Messages: 262
Registered: January 2006
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Senior Member |
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I'm new to DBA & I'm very confused with the concept of Checkpoint occurrence and SCN.Can anyone explain me these concepts briefly in layman terms?
Please anyone help me in clearing this concept as soon as possible,I'm very much confused.
[Updated on: Fri, 17 February 2006 00:34] Report message to a moderator |
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| Re: Checkpoint & SCN [message #159664 is a reply to message #159122] |
Mon, 20 February 2006 19:06  |
lazycat
Messages: 57
Registered: November 2005
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Member |
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Oracle Checkpoint
A checkpoint performs the following three operations:
1. Every dirty block in the buffer cache is written to the data files. That is, it synchronizes the datablocks in the buffer cache with the datafiles on disk.
It's the DBWR that writes all modified databaseblocks back to the datafiles.
2. The latest SCN is written (updated) into the datafile header.
3. The latest SCN is also written to the controlfiles.
The update of the datafile headers and the control files is done by the LGWR(CKPT if CKPT is enabled). As of version 8.0, CKPT is enabled by default.
Events that trigger a checkpoint
The following events trigger a checkpoint.
* Redo log switch
* LOG_CHECKPOINT_TIMEOUT has expired
* LOG_CHECKPOINT_INTERVAL has been reached
* DBA requires so (alter system checkpoint)
Additionally, if a tablespace is hot backuped, a checkpoint for the tablespace in question is taking place.
While redo log switches cause a checkpoint, checkpoints don't cause a log switch.
Time and SCN of last checkpoint
The date and time of the last checkpoint can be retrieved through checkpoint_time in v$datafile_header
The SCN of the last checkpoint can be found in v$database.checkpoint_change#.
Size of redo log
If the size of the redo log is to small, the performance of the checkpoint will not be optimal. This is the case if the alert.log contains messages like Thread .. cannot allocate new log....
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