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Re:RE: Not 100% clear on archive switching..

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Date: Wed, 27 Dec 2000 09:16:42 -0500
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    Let me see if I can explain this in a clear way & the way I understand it. Now I work mainly with HP-UX, but I believe the process is the same on all platforms.

    LGWR writes to the online redo log files in a sequential manner, log #1 followed by log #2 and so forth. (I always like at least 3 online log files.)

    When LGWR has filled an online redo log it signals for a checkpoint, along with any other checkpoint settings in the init.ora file, and a "log switch". If the database is in "archivelog" mode this log switch causes ARCH to 'copy' the online redo log to the archive log destination. In the process it creates a file name in the archive destination that is whatever you put as the archive log format + a sequence number. When it finishes copying the online redo log it updates the files status in dba_log_files, or more likely the underlying V$ table which is actually stored in the control file and it becomes available for LGWR to reuse which means that the data therein is overwritten, not that the file is erased & re-created or even resized, although I've got some questions on that score too. Most of my archive redo logs are 10M in size. But every so often I see one that is less than 10M.

    If the database has only the required 2 online redo logs and/or it's so busy that ARCH is getting close to 0 CPU time, or the archive log destination fills up, LGWR will continue filling online redo logs in sequence until there are no more available entries in dba_log_files. At this point in time the database ceases all activity (you end up with the ORA-00257 error message when logging on or trying any kind of statement) until al least one log file is released by ARCH. Thereafter things can get a little sporatic for a bit until ARCH catches up.

    Hope this helps. BTW: Do not, like an associate did, do a kill -9 on ARCH. It will die, the database will continue to run for a little bit, and you'll end up with one heck of a mess.

Dick Goulet

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Subject: RE: Not 100% clear on archive switching.. Author: Shawn Ferris <> Date: 12/26/2000 3:40 PM

> It seems to me that Oracle switches redo log from the full to the empty
> archiving purpose.

Well, the log may or may not be empty but you get the idea. I'm pretty sure it doesn't actually clear the file, it just overwrites it when available. (archived in archivelog mode)  

> Now the background process "ARCH" is in charge with the archiving
> process. Would you explain more?

Yes.. I'm not asking about online redo, I want to clarify my understanding of the archiving process and the order in which redo is archived to disk depending on the value of log_archive_max_processes.  

> And again I might not be completely right, but
> LOG_ARCHIVE_MAX_PROCESSES is targetting le number
> of processes, not redo logs.

Yes.. log_archive_max_processes determines how many ARCH processes can run in parallel. Each of which works on an single redo log to archive. (at least that is what I'm verifying.)

Shawn M Ferris
Oracle DBA - Time Warner Telecom  

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Author: Shawn Ferris

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Received on Wed Dec 27 2000 - 08:16:42 CST

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