In a metalink note, one says
"
Oralce writes the dirty buffers to disk only on certain conditions :
A shadow process must scan more than one-quarter of the db_block_buffer
parameter.
- Every three seconds.*
- When a checkpoint is produced.

A checkpoint is realized on five types of events:

- At each switch of the redo log files.
- When the delay for LOG_CHECKPOINT_TIMEOUT is reached.
- When the size in bytes corresponding to :
(LOG_CHECKPOINT_INTERVAL* size of IO OS blocks)
is written on the current redo log file.
- Directly by the ALTER SYSTEM SWITCH LOGFILE command.
- Directly with the ALTER SYSTEM CHECKPOINT command.


During a checkpoint the following occurs:
- The database writer (DBWR) writes all modified database
blocks in the buffer cache back to datafiles,
- Log writer (LGWR) updates both the controlfile and
the datafiles to indicate when the last checkpoint
occurred (SCN)**
"

So, I am a little bit confused (*, **):

* it is said that every 3s, Oracle writes dirty buffers to disk. So a checkpoint is occured every 3s or what ?
and why there is the file LOG_CHECKPOINT_TIMEOUT ?

** SCN is related to commited transaction, is there another SCN for checkpoint ?...


Thanks a lot ...

Sofiane