Re: Does an ALTER SYSTEM CHKPOINT write all dirty buffers to datafiles ?

joel-garry_at_home.com (Joel Garry) wrote in message news:<91884734.0307141442.7b7ff68a_at_posting.google.com>...
> spendius_at_muchomail.com (Spendius) wrote in message news:<aba30b75.0307140556.6a8baf93_at_posting.google.com>...
> > Does this command really ascertain that *all* modified buffers
> > will be written out to disk ??
> >
> > I'd just like to make sure (the doc. is not very clear about
> > it, and I found different answers to this question -according to
> > some it does write all dirty buffers, according to other people
> > it only updates the SCN in the datafiles' headers...-).
> >
> > Thanks.
> > Spendius
>
> If all it did was update the datafiles' headers, you'd never see
> "Checkpoint not complete" errors. Google for that.
>
> The idea of checkpointing is to be sure redo logs are written. Redo
> logs are the Achilles' Heel of Oracle. While nothing besides
> performance is likely to be affected if you are getting "Checkpoint
> not complete" errors, if your system goes down without finishing the
> checkpoint, you've quite possibly set up to lose stuff when instance
> recovery takes place on the next startup.
>

Hi Joel,

Couple of points.

The idea of checkpointing is to have a consistent point of time from which Oracle can perform instance recovery with the knowledge that all changes in the buffer cache prior to the checkpoint have been written to the OS. It has nothing to do with ensuring redo logs are written to except to ensure that the current contents of the redo buffer are flushed to the OS as well (to ensure all changes written to be DBWR are safely recorded in the redo logs).

If the system goes down without finishing the checkpoint it does not mean "you are set up to lose stuff". It simply means Oracle can't guarantee that all changes associated with a previous redo log have been written to disk therefore the previous redo log would be required for instance recovery as well as the current log. *No commited data can be lost* as Oracle guarantees all commited data to be written to the redo log files. If a redo log to be overwritten has not yet had it's corresponding checkpoint complete, Oracle simply waits until the checkpoint is complete to ensure that instance recovery can always be completed. Oracle prefers to hang the system rather than risk loss of data.

Redo logs are only the " Achilles' Heel " if you lose all members of the current redo log group, which is quite a different scenario (and one which should never occur if the redo logs are configured appropriately).

Cheers

Richard Received on Mon Jul 14 2003 - 22:21:46 CDT