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Don Burleson Blog 







Alter system switch logfile tips

Oracle Database Tips by Donald BurlesonMay 16, 2015

Question:  I joined a new company that backs up the archive logs to disk every 15 minutes via a crontab job, and forces a log switch with "alter system switch logfile".

This creates a new archive log.  Each log is from 11K to 45mb.  I have a feeling it is a over do - and I would like to do the archive log backup every hour.  That would produce about 24 logs a day rather than 100 logs.  Is there any benefit to doing a "alter system switch logfile"?

Answer:  As you noted, the "alter system switch logfile" command forces a log switch.  It will produce an archived redo log that is smaller than the size dictated by the size of the log_buffer parameter, which governs the size of the redo log RAM buffer.

Time-based redo log switching results in too many archived redo logs, all of varying sizes.  Sometimes, having consistent redo log file sizes is handy, such as needing to know when the archiver process (ARCH) is done writing a redo log file.  If you know the final files size, you know when it is done.

I do an "alter system switch logfile" when doing backups to flush out the last of the redo data from the log_buffer.  The "alter system switch logfile" is also used when re-sizing online redo logs:

alter database drop logfile group 3;
ALTER DATABASE ADD LOGFILE THREAD 1 GROUP 3 '/oracle/oradata/CURACAO9/redo03.log' size 500m reuse;
ALTER DATABASE ADD LOGFILE member '/oracle/oradata/CURACAO9/redo03b.log' reuse to GROUP 3;
alter system switch logfile;
alter system checkpoint;
alter database drop logfile group 2; -- and do group 2 etc.



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