Re: LGWR special commit/checkpoint records

From: Joel Garry <joelga_at_pebble.ml.org>
Date: 4 May 1998 17:22:14 -0700
Message-ID: <6ilm3m$cbv$1@pebble.ml.org>

In article <6ibqdg$n7b$1_at_reader1.reader.news.ozemail.net>, Doug Carter <dcarter_at_tui.com.au> wrote:
>This is for the more technical of you. I had a
>rather good question from a database user today
>that I would like verified. (Replies direct to
>me if possible).
>
>He understood that you can recover to a point in
>time using redo data. What he wanted to know
>was in what format this timestamp is kept. i.e.
>did it include Timezone information or without etc.
>
>I know that the redo log contains two special
>records - written at commit and checkpoints, and
>that each redo log block contains an 18 byte header.
>I assume that the timestamp sits in the special
>records for obvious reasons.
>
>Now the normal date datatype in oracle is stored
>in a 7 byte field as an excess 100 number. This
>would seem unsuitable for something which is used
>for a point in time recovery - especially around
>day light savings!

The Oracle Backup & Recovery Handbook has a good description of this problem. Oracle recovery scans forward based on SCN and the timestamp, which is apparently just a normal timestamp. So it winds up depending on whether there were any transactions during the time just before the clock was changed as to whether you can recover to a point just after the clock was changed. A complete recovery will recover all transactions.

The answer is to take a complete backup of the database just after changing the clock, to avoid later transactions with earlier timestamps.

>
>I think I heard once that oracle can use a value of
>seconds after a point in time around 1972 (sorry,
>can not remember the correct term for it, but it
>is not Oracle specific) based on GMT. I assume that
>Oracle uses this.

Naw, you must be thinking of RSTS or VMS or unix or something.

>
>Can someone please fill in the assumptions?
>
>Much appreciated,
>
>Doug Carter
>Database Administrator
>TUI Consulting
>
>

--
These opinions are my own and not necessarily those of Information Quest

jgarry@eiq.com                           http://www.informationquest.com

http://ourworld.compuserve.com/homepages/joel_garry "See your DBA?" I AM the @#%*& DBA! Received on Mon May 04 1998 - 19:22:14 CDT