Re: LGWR & Checkpoints Quick Q

LGWR:
Yes the LGWR will write uncommitted changes into the redo logs. Think about it, if log writer were do things only once committed and 30Mb of changes are made to the database and then committed... having to suddenly write the 30Mb out to the redo logs would be a huge bottle neck. Rather LGWR writes out the changes as they happen, using the criteria you stated, then when a commit happens it also puts that information into the log.

The redo logs are used for recover of the system, during a recovery Oracle will roll-forward the changes found until it hits the end of all the redo and then rollback the uncommitted changes. It does this by re-building the rollback segment information during the recovery.

The rollback segments are used by transactions to get the information needed to present a time consistent query and also to roll back any statements that either fail or issue a implicit rollback.

CPKT:
The CKPT does update the datafile headers with the latest SCN.

For more information you could try www.ixora.com.au, http://www.jlcomp.demon.co.uk, www.hjrdba.com, http://asktom.oracle.com. There are also some good books out there, specifically I'd recommend Oracle books from the O'Reilly series and "Practical Oracle 8i" by Jonathan Lewis as a good place to start.

Kind Regards

Fraser McCallum
MVP Oracle Administration
www.brainbench.com

"gl" <jogret_at_hotmail.com> wrote in message news:3c8e6f6d$1_at_hp176.detroit.deco.com...
> I'm reading the concepts manual (8.1.6) about LGWR and checkpoints and I'm
> getting a little confused:
>
> LGWR:
> Besides writing commited records, LGWR will write redo log buffers every 3
> sec., 1/3 redo log buffer full, and when DBWn writes dirty blocks to disk
> (checkpoint). Are these writes un-commited data? If so, isn't that the
job
> of rollback segments?
>
> CKPT:
> Checkpoint is updating datafile headers as to the most recent checkpoint.
> Is this based on time (it couldn't be based on SCN)?
>
> Is there any other source I can use other than the concepts manual to find
> out more info. on CKPT and LGWR processing?
>
> Thanks in advance.
>
>
Received on Tue Mar 12 2002 - 18:02:57 CST