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Data Guard is Oracle's database replication product used for creating
and maintaining "standby" disaster recovery databases. This is done by leveraging
Oracle arhived redo logs generated when the database is in archive mode.
The archived redo logs from the primary database are periodically sent to a read-only
standby database that applies each archived redo log to itself each time ones comes over.
Standby databases come in 2 varieties: Physical and Logical.
A physical standby database is identical block for block to the primary database,
hense the standby system will need to be designed effectively identical to the
primary database. A logical standby database can be very diffrent in design to the
primary database so long as it has suffient resources to contain the primaries data.
In the case of a physical standby archived redo logs can be directly applied to the standby
database whereas the archived redo logs when applied to a logical standby need to be ripped
apart into SQL statements and applied one by one. The options allow for
a flexable architecture based on your enviroments avalible resources.
The standby database sits idle most of the time in read-only mode accepting
archive redo logs from the primary. This makes the standby system idea for running
reports and intensive queries. If the primary database fails for some reason,
the standby can simply be restarted in read-write mode and used untill the primary comes
back online.
For small organizations where complex architectures for RAC and data warehousing
aren't practical a standby database using Data Guard can fill a variety of
needs all at one time.
A brief aside, I'll note that some companies actually use RMAN as a replication
scheme by immediately recovering a backup to a "standby" system and using
it for reports and read-only access just as if they were using Data Guard.
The advantage of RMAN over Data Guard for replication is that you always
are testing the validity of your backup system. The downside to using
RMAN is that it's not at all effectient and you typically don't run RMAN backups
more than once a day making it a poor HA solution by itself. There are a variety
of opinions on this practice but just know that it isn't unheard of.
For more information about Flashback check out the book
Oracle Database 10g High Availability with RAC,
Flashback and Dataguard and the Oracle Data Guard Concepts and Administration
manual:
http://download-west.oracle.com/docs/cd/B14117_01/server.101/b10823/toc.htm
Next: OLTP
Up: Loose Ends
Previous: Oracle Flashback
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2005-02-10