MirrorPeriod defines the period of time when indexer is allowed to reuse previously mirrored documents as crawler cache instead of real downloading (to reduce Internet traffic).
Note: Using mirrors as crawler cache can be useful when you are doing experiments with mnoGoSearch to find the best configuration that suites your need.
The default value of is -1, which means that the mirrored files are not used as crawler cache.
See the Period command description for time format.