Instance Needs Recovery: Introduction (2/3)

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Closing a Redo Log file

There is always a CURRENT Redo Log file (one for each Instance of RAC).
When Oracle begin to write the CURRENT Redo Log file, it set the Low SCN (V$LOG.FIRST_CHANGE#) to the current SCN of the database and set the High SCN (V$LOG.NEXT_CHANGE#) to 281474976710655. Think this number as a infinite value.

Oracle do this because it don't know the final value of the High SCN,

When there is a "Log Switch (it appen wheh the CURRENT Redo Log is full or a DBA run "ALTER SYSTEM SWITCH LOGFILE statement) Oracle set the V$LOG.NEXT_CHANGE# column  to current SCN +1, and than begin to use the next Redo Log file.

The operation that set the V$LOG.NEXT_CHANGE# to a finite value, is called "closing the Redo Log file".

Archived Log

Some definition:

"A log switch is the event during which LGWR stops writing to one online redo log group and starts writing to another" (1). "The background process ARCn automates archiving operations when automatic archiving is enabled" (2)

"An archived redo log file is a copy of one of the filled members of a redo log group". "The process of turning online redo log files into archived redo log files is called archiving" (2)

"When running in ARCHIVELOG mode, the log writer process (LGWR) is not allowed to reuse and hence overwrite an online redo log group until it has been archived" (2)

When the log switch occur
(i) LGWR starts writing to another one
(ii) the ARCn process automates archive in backup the CURRENT redolog (3)

so, a user session not need to wait the arichive finish



(1) https://www.bnl.gov/phobos/Detectors/Computing/Orant/doc/database.804/a58397/ch5.htm
(2) https://docs.oracle.com/cd/B19306_01/server.102/b14231/archredo.htm
(3) http://www.datadisk.co.uk/html_docs/oracle/redo.htm
(4) Master Note: Troubleshooting Redo Logs and Archiving (Doc ID 1507157.1)

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