Re: Does a commit cause a checkpoint?

On Thu, 19 Jun 2003 03:15:33 GMT, Hans Forbrich <forbrich_at_telusplanet.net> wrote:

A commit statement does not cause an incrrement in SCN. It causes the LGWR to flush out redo logs. I hope this answer the question.

>Peter wrote:
>
>> On Thu, 19 Jun 2003 00:41:45 GMT, Hans Forbrich
>> <forbrich_at_telusplanet.net> wrote:
>>
>> A logswitch causes a checkpoint which causes LGWR to flush the redo
>> log buffer, but no change in SCN
>
>>
>> A checkpoint causes the DBWn to write to file, and also causes the
>> LGWR to flush the redo buffer, but no change in SCN
>
>Basically correct .... the stuff below is from the various documentation to support this and
>expand into exceptions, strange conditions, etc.
>
>/Hans
>
>From "Oracle9i Database Performance Tuning Guide and Reference Release 2 (9.2) Part Number
>A96533-02", chapter 17, (where you can learn a bt more about the impact to a running database)
>... in section "
>Checkpointing and Cache Recovery"
>
>>>>>>>>>>>
>Periodically, Oracle records a checkpoint. A checkpoint is the highest system change number
>(SCN) such that all data blocks less than or equal to that SCN are known to be written out to
>the data files. If a failure occurs, then only the redo records containing changes at SCNs
>higher than the checkpoint need to be applied during recovery.
><<<<<<<<<<
>
>----------
>
>From Oracle9i Database Concepts, Release 2 (9.2), Part Number A96524-01, Chapter 1, in the
>section about Process Architecture, we see
>
><<<<<<<<
>Checkpoint (CKPT)
>
>At specific times, all modified database buffers in the SGA are written to the datafiles by
>DBWn. This event is called a checkpoint. The checkpoint process is responsible for signaling
>DBWn at checkpoints and updating all the datafiles and control files of the database to indicate
>the most recent checkpoint.
>>>>>>>>>
>
>-------------
>Also note - from the "Oracle9i Backup and Recovery Concepts, Release 2 (9.2), Part Number
>A96519-01" the glossary has the following very useful definitions:
>
>1) checkpoint
>A data structure that defines an SCN in the redo thread of a database. Checkpoints are recorded
>in the control file and each datafile header, and are a crucial element of recovery.
>
>2) database checkpoint
>The thread checkpoint that has the lowest SCN. The database checkpoint guarantees that all
>changes in all enabled threads prior to the database checkpoint have been written to disk.
>
>See Also: checkpoint
>
>3) datafile checkpoint
>The checkpoint structure stored in the header of each datafile. All redo in all threads prior to
>the datafile checkpoint SCN is guaranteed to have been saved to the datafile.
>
>4) offline-end checkpoint
>The SCN that specifies when a datafile was brought online after being offline, or made
>read/write after being read-only. This SCN is stored in the control file and is the last SCN in
>the offline range. The offline-end checkpoint is important because it indicates that changes
>after this SCN are required to recover the datafile.
>
>5) thread checkpoint
>A type of checkpoint stored in the control file indicating that all changes to online datafiles
>in a given thread prior to the checkpoint SCN have been saved to disk. Oracle updates the thread
>checkpoint every time an instance checkpoints its thread.
>
>----------------
>
>And finally I suggest you read the "Oracle9i Database Concepts, Release 2 (9.2) Part Number
>A96524-01", Chapter 8 "Process Architecture" which has some decent diagrams and an excellent
>section on the Log Writer which support and expands this.
Received on Thu Jun 19 2003 - 01:03:49 CDT