Thanks for your answer, will treasure your lessons.
bye
Francesco
"Mark D Powell" <Mark.Powell_at_eds.com> ha scritto nel messaggio
news:2687bb95.0306050612.396f944e_at_posting.google.com...
> "Jan Gelbrich" <j_gelbrich_at_westfalen-blatt.de> wrote in message
news:<bbmu76$b9886$1_at_ID-152732.news.dfncis.de>...
> > "F.Marchioni" <NOSPAMfmarchioni_at_libero.it> schrieb im Newsbeitrag
> > news:AoCDa.61980$Ny5.1875602_at_twister2.libero.it...
> > > Hi all,
> > > please forgive the beginner's question, I'm just studying something
> > > about Oracle architecture. I'm just wondering why Oracle needs
> > > both redo logs and rollback segments to keep track of transactions...
> > > reading the manual they seem to pursue a quite similar job....anybody
> > > can give me an explanation ?
> > > Thanks a lot
> > > Francesco
> > >
> > >
> > Hi Francesco,
> >
> > redo logs and rollback segments have _opposite_ pursposes:
> >
> > redo log: _re_do, e.g. in the case of a disk crash (that is why redo
logs
> > should be mirrored on different disks)
> > rollback: _un_do transactions that You do not want to commit;
> >
> > Do not worry; when I was a newbee, I was confused just the same ;-)
> >
> > hth, Jan
>
> Oracle could have chosen to perform rollback using the online redo
> logs but that would create a conflict from the log writer wanting to
> write sequentially to the end of the file and the need to do random IO
> for rollback. So there is a performance benefit from separating the
> rollback and logging activities.
>
> And while rollback segments do provide the necessary information to
> roll transactions back their most commonly used functionality in
> Oracle is that it is from the rollback segments that Oracle builds
> read consistent views of data. Whenever you issue a SQL statement in
> Oracle all the data returned to that cursor is from the same point in
> time. So if your query starts at exactly 9 AM and takes 5 minutes to
> complete all the rows returned to your query look as they were at 9
> AM. Changes applied between 9 and 9:05 to the rows your query select
> are "rolled back" to provide your query a time consistent view of the
> data. Using rollback segments this way allows Oracle to never show
> the user uncommited changes in response to a query while avoiding the
> necessity to use read locks to protect the data. Performing this
> activity using the online redo logs would have really created an IO
> contflict between the tasks, hence the wise decision to keep these
> function separate.
>
> Jan noted the following in his post but because these are often
> confused
> Online logs => redo
> rollback segments => undo
>
> I hope this additional information helps your understanding. The
> Concepts manual does a pretty good job covering on these features so
> you may want to review them there.
>
> HTH -- Mark D Powell --
Received on Thu Jun 05 2003 - 09:48:02 CDT
