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<h2 class="title" style="clear: both"><a id="hotfailover"></a>Using Hot Failovers</h2>
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<p>
You can maintain a backup that can be used for failover purposes.
Hot failovers differ from the backup and restore
procedures described previously in this chapter in that data
used for traditional backups is typically copied to offline storage.
Recovery time for a traditional backup is determined by:
</p>
<div class="itemizedlist">
<ul type="disc">
<li>
<p>
How quickly you can retrieve that storage media.
Typically storage media for critical backups is moved
to a safe facility in a remote location, so this step can
take a relatively long time.
</p>
</li>
<li>
<p>
How fast you can read the backup from the storage media
to a local disk drive. If you have very large backups,
or if your storage media is very slow, this can be a
lengthy process.
</p>
</li>
<li>
<p>
How long it takes you to run catastrophic recovery
against the newly restored backup. As described earlier
in this chapter, this process can be lengthy because
every log file must be examined during the recovery
process.
</p>
</li>
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<p>
When you use a hot failover, the backup is maintained
at a location that is reasonably fast to access. Usually, this
is a second disk drive local to the machine.
In this situation, recovery time is very quick
because you only have to reopen your
environment and database, using the failover environment
for the environment open.
</p>
<p>
Hot failovers obviously do not protect you from truly
catastrophic disasters (such as a fire in your machine room)
because the backup is still local to the machine. However,
you can guard against more mundane problems (such as a broken
disk drive) by keeping the backup on a
second drive that is managed by an alternate disk controller.
</p>
<p>
To maintain a hot failover:
</p>
<div class="orderedlist">
<ol type="1">
<li>
<p>
Copy all the active database files to the failover
directory. Use the <span><b class="command">db_archive</b></span>
command line utility with the
<tt class="literal">-s</tt> option to identify all the active
database files.
</p>
</li>
<li>
<p>
Identify all the inactive log files in your production
environment and <span class="emphasis"><em>move</em></span> these to the failover
directory. Use the <span><b class="command">db_archive</b></span>
command with no command line options to obtain a list
of these log files.
</p>
</li>
<li>
<p>
Identify the active log files in your production
environment, and <span class="emphasis"><em>copy</em></span> these to the
failover directory. Use the
<span><b class="command">db_archive</b></span> command with the
<tt class="literal">-l</tt> option to obtain a list of these
log files.
</p>
</li>
<li>
<p>
Run catastrophic recovery against the failover
directory. Use the <span><b class="command">db_recover</b></span>
command with the <tt class="literal">-c</tt> option to do
this.
</p>
</li>
<li>
<p>
Optionally copy the backup to an archival location.
</p>
</li>
</ol>
</div>
<p>
Once you have performed this procedure, you can maintain an
active hot backup by repeating steps 2 - 5 as often
as is required by your application.
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
If you perform step 1, steps 2-5 must follow in order to
ensure consistency of your hot backup.
</p>
</div>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p>
Rather than use the previous procedure, you can use the <span><b class="command">db_hotbackup</b></span> command line utility
to do the same thing. This utility will (optionally) run a checkpoint and then copy all necessary files to a target
directory for you.
</p>
</div>
<p>
To actually perform a failover, simply:
</p>
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<ol type="1">
<li>
<p>
Shut down all processes which are running against the original environment.
</p>
</li>
<li>
<p>
If you have an archival copy of the backup environment, you can optionally try copying the remaining
log files from the original environment and running catastrophic recovery against that backup
environment. Do this <span class="emphasis"><em>only</em></span> if you have a an archival copy of the backup
environment.
</p>
<p>
This step can allow you to recover data created or modified in the original environment, but which
did not have a chance to be reflected in the hot backup environment.
</p>
</li>
<li>
<p>
Reopen your environment and databases as normal, but use
the backup environment instead of the production
environment.
</p>
</li>
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