Re: DBWR and LGWR processes, theoretical question

"tojo" <TomJordanTojo_at_hotmail.com> wrote in message news:MPG.182b644242d849b89896c6_at_news.t-online.de...
> In article <3DC132C2.2030905_at_web.de>, mig-sm_at_web.de says...
> > Hi,
> >
> > i'm currently 'upgrading' my knowledge of Oracle from version 7.x to
> > 9.2. Reading through the Database Concepts manual, i found that Oracle
> > uses write ahead logging. If i understand it correctly, that means that
> > there could be a time gap between writing committed data changes to the
> > tablespace files by the DBWx process and writing them to the redo log
> > files by the LGWR process (as far as i can see now, there is no change
> > since Oracle 7.x on this point).
> >
> > Now my question:
> > Take a system that modifies data heavily. To make it simple, let us use
> > only one transaction changing a lot of data (Example: In my last project
> > i had the requirement to insert 650 MB of raw data in up to 160000 rows
> > within a single transaction).
> >
> > So, what is happening?
> > - LGWR writes the committed statement to the redo log file. Due to the
> > write-ahead logging it finishes faster than the DBW0 - DBWx processes.
> > - DBW0 - DBWx are processing their batch to write the changes to the
> > tablespaces (which may take a little bit of time due to the data
> > volume).
> >
> > Now we have the state, that a committed transaction only is stored in
> > the redo log file but not necessarily is stored completely in the
> > tablespace. As far as i understand the checkpoint is not reached (i
> > assume, a usual state in such an environment).
> >
> > At this point of time the server crashes and let us assume the last redo
> > log file is corrupt (simple example, no redo log file groups).
> >
> > I think that the committed transaction cannot be recovered by Oracle. Am
> > i right?
> >
> > What if i use two redo log file groups. Now i have at least one
> > corrupted redo log file. Does Oracle check if a redo log file is
> > correct? How is it done?
> >
> >
> That's why you should always multiplex your redo logs (i.e. more than
> one group).

Just being picky here, but since this is technology, it's important to use words with the meaning with which they are normally invested. Multiplexing does NOT mean 'more than one group'. Every Oracle database on the planet must have at least two GROUPS, anyway... but that does nothing to protect each group, since each is entirely independent of the other,

Multiplexing your redo logs means adding additional MEMBERS to each group. Two groups, two members each = 4 redo log files. Having a minimum of two groups is compulsory. Having multiple members within a group isn't.

Groups and members are quite different things, in other words.

Regards
HJR
>Besides, you're talking about two catastrophies occurring
> simultaneously (server crash / redo log file corrupt). The one doesn't
> necessarily cause the other. There's a different recovery method for
> each case (see Backup and Recovery docs).
>
> -- Tom
Received on Thu Oct 31 2002 - 14:11:37 CST