RMAN - ¿Qué es un SCN?

An SCN in Oracle is nothing more than Oracle's clock: a chronology of changes.

It is this chronology that affords Oracle the ability to perform recovery with exact precision.

There are many types of events that trigger many types of checkpoints. But, for the sake of this discussion consider the global checkpoint triggered by a log switch. Upon this triggering event Oracle "notes" the current SCN. This is the SCN that will be written to the data file headers and their corresponding control file entries once Oracle has written ALL blocks up to and including that SCN to said files. At that point Oracle can assert with 100% certitude that the data file(s) are consistent with respect to that SCN - or simply that change-point in time. This doesn't mean all of the data written is committed or that there aren't blocks in the data files with higher SCNs by the time the checkpoint completes. It only means that Oracle can guarantee all changes, up to and including the checkpoint SCN, are written to the files.

Fuentes:

• http://www.stanford.edu/dept/itss/docs/oracle/10g/server.101/b10735/setup.htm
• http://www.orafusion.com/art_rman2.htm
• http://www.oracle.com/technology/deploy/availability/pdf/rman_10g_fov.pdf
• http://blogs.ittoolbox.com/bi/confessions/archives/how-oracle-works-12-a-note-about-rman-10225