If a session uses a Lookup transformation, you can configure the transformation to use a persistent lookup cache. With this option selected, the PowerCenter Integration Service process saves the lookup cache to disk the first time it runs the session, and then uses this lookup cache during subsequent session runs. By default, the DTM creates the index and data files in the directory configured for the $PMCacheDir service process variable. If you do not name the files in the transformation properties, these files are named PMLKUP*.idx and PMLKUP*.dat.