Database checkpoint

Database checkpoints are closely tied to redo log file switches. A checkpoint is an event that
flushes the modified data from the buffer cache to the disk and updates the control file and datafiles.
The CKPT process updates the headers of datafiles and control files; the actual blocks are
written to the file by the DBWn process. A checkpoint is initiated
When the redo log file is filled and a log switch occurs.
When the instance is shut down with NORMAL, TRANSACTIONAL, or IMMEDIATE.
When a tablespace status is changed to read-only or put into BACKUP mode.
When other values specified by certain parameters (discussed later in this section) are
reached.
You can force a checkpoint if needed, as shown here:
ALTER SYSTEM CHECKPOINT;
Forcing a checkpoint ensures that all changes to the database buffers are written to the datafiles
on disk.
Another way to force a checkpoint is by forcing a log file switch:
ALTER SYSTEM SWITCH LOGFILE;

Automatic Checkpoint Tuning
===========================
Oracle Database 10g supports automatic checkpoint tuning. It is enabled if
fast_start_mttr_target is explicitly set to an non-zero value, or
if fast_start_mttr_target is not set at all.
It is an advancement over the MTTR related parameter introduced in
earlier versions. The idea is to use the periods of low I/O usage
to advance checkpoints and therefore improve availability.

How it works
============
Enabling fast-start checkpointing increases the average number of writes
per transaction that DBWn issues for a given workload. However, if the
system is not already near or at its maximum I/O capacity, then
fast-start checkpointing has a negligible impact on performance

How to monitor
==============
View V$MTTR_TARGET_ADVICE will give information on the additional I/O's
on various values of FAST_START_MTTR_TARGET.

PROs and CONs:
=============
- FAST_START_MTTR_TARGET is set to a low value :
Fast-start checkpointing is more aggressive.
The average number of writes per transaction that DBWn issues
is higher in order to keep the thread checkpoint sufficiently
advanced to meet the requested MTTR.
- FAST_START_MTTR_TARGET is set to a high value:
Fast-start checkpointing in less aggressive, and the average
number of writes per transaction that DBWn issues is lower.