cdm/src/main/java/ucar/nc2/ncml/Aggregation.java
static public void setPersistenceCache(DiskCache2 dc) {
    diskCache2 = dc;
    if (diskCache2 != null)
      diskCache2.setAlwaysUseCache(true); // the persistence cache file has same name as the ncml - must put it into the cache else clobber ncml  7/31/2014
  }
static public void setPersistenceCache(DiskCache2 dc) {
    diskCache2 = dc;
    if (diskCache2 != null)
      diskCache2.setAlwaysUseCache(true); // the persistence cache file has same name as the ncml - must put it into the cache else clobber ncml  7/31/2014
  }
this is where  persist() reads/writes files