SCN and Log Sequence Number

What are the differences and similarities (if any) between SCN and and Log Sequence Number?

gaurav_ry
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Sep 3rd, 2008
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sagar verma

Sep 7th, 2011

The SCN plays an important role to determine if the database is in a consistent state when the database is brought online. SMON checks the SCN in all datafile headers when the database is started. Everything is OK if all of these SCNs matches the SCN found in the controlfile. If the SCNs don't match, the database is in an inconsistent state.

This number is written into the header of each datafile.

SCN = system change number is assigned(& incremented) everytime someone commits(ie transaction completes)

Checkpoint number is the SCN number at which all the dirty buffers are written to the disk, there can be a checkpoint at object/tablespace/datafile/database level.

Whenever Oracle (or more precisely, the log writer process) writes to another online redo log file group (also referred to as log switch), the log sequence number increments by one.