Re: Oracle Instance Recovery (I don't get it :)

QTFM: "The Redo Log and Rolling Forward

The redo log is a set of operating system files that record all changes made to any
database buffer, including data, index, and rollback segments, whether the changes
are committed or uncommitted. The redo log protects changes made to database buff-ers
in memory that have not been written to the datafiles.

The first step of recovery from an instance or disk failure is to roll forward, or reap-ply
all of the changes recorded in the redo log to the datafiles. Because rollback data is also recorded in the redo log, rolling forward also regenerates the corre-sponding
rollback segments.

Rolling forward proceeds through as many redo log files as necessary to bring the
database forward in time. Rolling forward usually includes online redo log files

and may include archived redo log files.

After roll forward, the data blocks contain all committed changes as well as any

uncommitted changes that were recorded in the redo log.

Rollback Segments and Rolling Back

Rollback segments record database actions that should be undone during certain database operations. In database recovery, rollback segments undo the effects of

uncommitted transactions previously applied by the rolling forward phase.

After the roll forward, any changes that were not committed must be undone. After
redo log files have reapplied all changes made to the database, then the correspond-ing
rollback segments are used. Rollback segments are used to identify and undo transactions that were never committed, yet were recorded in the redo log and applied to the database during roll forward. This process is called rolling back."

Andrey Dmitriev wrote:

> Now I am really confused. I thought so too, but as I was reading
> Oracle 8 Tuning (Oracle series) I have noticed that the author(s) say
> that Redo Logs contain only committed information. Does anybody else
> have an opinion? Thanx. Meanwhile I think I am going to order Oracle 8
> Architecture from bn or amazon :)
>
> On Sun, 20 Jun 1999 23:26:53 +0200, "Sybrand Bakker"
> <postbus_at_sybrandb.demon.nl> wrote:
>
> >Hi Andrey,
> >
> >Rollback segments are protected by the redo log files too, so rollback data
> >is in the redo log. This makes sure you can rollback uncommitted
> >transactions.
> >
> >Hth,
> >
> >Sybrand Bakker, Oracle DBA
> >
> >Andrey Dmitriev wrote in message <376d2dfc.6908664_at_news.earthlink.net>...
> >>Hi,
> >>
> >>I don't completely understand what is going on during Oracle Instance
> >>Recovery. The redo logs are applied (roll forward?) and then
> >>uncommited transactions are rolled back? But don't redo logs only have
> >>only commited transactions?
> >>
> >>Thanx
> >>
> >
> >
Received on Sun Jun 20 1999 - 19:02:40 CDT