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Oracle Failover Options

Oracle Tips by Burleson Consulting

For cost issues see related notes on Oracle licensing Pricing and
Oracle standby database license requirements
Many cost-conscious Oracle shops as how they can get Oracle failover as cheaply as possible. 

Oracle has a wealth of failover options, each with its own costs and benefits:

Manual failover: allows Oracle SE, but has data loss and up to 30 minutes to recover

Data Guard: Requires Oracle EE, but automates the failover process to reduce downtime
 
Replication: Multi-master replication can allows the standby database to be open and accepting transactions, with DML cross-feeding each instance.  This requires Oracle EE and is only for databases with low DML rates. 

Oracle Streams: High-speed replication from the Oracle log_buffer allows fast synchronization between servers.  Oracle Streams requires Oracle10g Enterprise Edition.

Let's take a closer look at the cheapest solution, Oracle manual standby database.

 

Manual standby database

 

If you want to save money by using Oracle Standard Edition which does not have Oracle Data Guard, multimaster replication and Oracle Streams, a manual approach might work for you.  In this approach you write a daemon process on the OS that detects when the Oracle ARCH process has completed writing an archived redo log. 

 

Then, the daemon uses FTP or rsh to move the archived redo to the standby database server where it is automatically accepted into the standby database because it remains in "recover database" mode.

 

The trick to this approach is to set the size of the online redo logs to minimize data loss.  This is done by examining the log switch rate for the Oracle database at its PEAK DML load.  For example, if you determine a size that has log switching every 5 minutes at peak load (with an average of, say 15 minutes, the maximum data loss is 15 minutes.

 

Many shops improve this mechanism with solid-state disk for the online redo logs and archived redo log filesystem, SSD can write up to 100 times faster, reducing recovery time by two orders of magnitude, reducing the maximum data loss from 15 minutes to less than one minute. 

 

Oracle Data Guard Failover Options

There are four methods for achieving failover technology within the Oracle software.  Each failover option has its own costs, advantages and disadvantages.
When choosing a failover option, the DBA must consider their tolerance for unplanned downtime as well as the cost per minute for downtime.

Each of these options will incur different downtimes, configuration costs and expenses.  Since Oracle introduced recovery products 12 years ago, their technologies have evolved significantly:

  • Traditional recovery (1990-1995): This recovery method requires restoration of failed database files and a roll-forward using Oracle?s Enterprise Backup Utility (EBU) or the Oracle8 Recovery Manager (RMAN) utility. This type of recovery could take several hours.
  • Manual Standby databases (1993-present) - Oracle7 introduced mechanisms that allow a standby database to be constantly in recovery mode and to be refreshed from Oracle's archived redo logs. In case of failure, the last redo log could be added to the standby database, and the database could be started in just a few minutes. Standby database is not an Oracle product, but a procedure that was used prior to Oracle Data Guard to create a standby database.  Oracle7 introduced mechanisms that allow a standby database to be constantly in recovery mode and to be refreshed from Oracle's archived redo logs. In case of failure, the last redo log could be added to the standby database, and the database could be started in just a few minutes.
  • Replication failover: Multi-master replication can allow the standby database to be open and accepting transactions, with DML cross-feeding each instance.  This requires Oracle Enterprise Edition and is only for databases with low DML rates.
  • Real Application Clusters  - The RAC architecture allows many instances to share a single database, but it avoids the overhead of RAM block pinging. RAC has also been enhanced to work with Oracle's Transparent Application Failover (TAF) to automatically restart any connections when an instance fails.
  • Oracle Streams - This is a high-speed replication solution that takes SQL directly from the log_buffer  RAM area and replicates transactions to a remote database.  During a server crash, transactions can be quickly redirected to the replicated system.
  • Oracle Data Guard - This is a free option with Oracle Enterprise Edition, and it provides an automated standby database.  Upon server failure, a series of database procedures synchronizes the standby database and opens it to accept connections. Oracle Data Guard is free with Enterprise Edition.

 

In sum, the standby database approach has these issues:

 

            a) Allows Oracle Standard Edition (far cheaper than enterprise edition).

 

            b) Requires a license for the standby database.

 

c) If standby database is geographically removed, this also provides disaster recovery.

 

d) Recovery can take up to 15 minutes.

 

e) If the instance fails, the last redo log remains in the log buffer and online redo log files.  This means that there might be a small data loss.

 

f) Requires manual intervention to open the surviving standby database and re-direct incoming requests (via tnsnames.ora or Apache) ton the new server.

 Costs for standby database licenses

This Oracle license pricing document (as of March 16, 2006), notes that mall standby and failover database must be fully licensed if they might be used as production:

"Standby – In this type of recovery, a copy of the primary database is maintained on a separate server at all times.

These systems are configured for disaster recovery purposes. If the primary database fails, the standby database is activated to act as the new primary database.

In this environment, both the primary and the standby databases must be fully licensed."

   
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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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