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DBWR and LGWR processes, theoretical question

From: Michael Gast <mig-sm_at_web.de>
Date: Thu, 31 Oct 2002 14:40:18 +0100
Message-ID: <3DC132C2.2030905@web.de>


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?

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Mit freundlichen Grüßen / Best Regards
Michael Gast
SEPP MED GmbH
Received on Thu Oct 31 2002 - 07:40:18 CST

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