Re: Does a commit cause a checkpoint?

Hi to all,

Commit doesn't cause a checkpoint itself. If it was so, there would not be a need for redo-logging.

There is an article of K Gopalakrishnan. It's published in Oracle Internals(http://www.oracle-internals.cc) magazine. Gopal explains the relatioship and differences between commit-SCN and SCN.

best regards...

Danisment Gazi Unal
http://www.ubTools.com
Web Based Oracle Products and Services

Peter <peter_at_nomorenewsspammin.ca> wrote in message news:<4am2fvgpjg9e1hak3t2bjflsihbdd0td9k_at_4ax.com>...
> On Thu, 19 Jun 2003 06:03:49 GMT, Peter <peter_at_nomorenewsspammin.ca>
> wrote:
>
> System change number (SCN)
> A stamp that defines a committed version of a database at a point in
> time. Oracle assigns every committed transaction a unique SCN
> This is from the manual and should answer the queston.
>
> >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 - 07:03:15 CDT