http://docs.splunk.com/Documentation/Splunk/latest/admin/inputsconf
In the inputs.conf use this:
ignoreOlderThan = <time window>
* Causes the monitored input to stop checking files for updates if their modtime has passed this threshold.
This improves the speed of file tracking operations when monitoring directory hierarchies with large numbers
of historical files (for example, when active log files are colocated with old files that are no longer
being written to).
* As a result, do not select a cutoff that could ever occur for a file
you wish to index. Take downtime into account!
Suggested value: 14d , which means 2 weeks
* A file whose modtime falls outside this time window when seen for the first time will not be indexed at all.
* Value must be: <number><unit> (e.g., 7d is one week). Valid units are d (days), m (minutes), and s (seconds).
* Default: disabled.
