@Override
    protected int getCacheSize(FileCache<FileData> fc) {
      return 1; // Each iteration cleans the file cache as a single unit (unlike the ORC cache).
    }
RW lock ensures we have a consistent view of the file data, which is important given that
we generate "stripe" boundaries arbitrarily. Reading buffer data itself doesn't require
that this lock is held; however, everything else in stripes list does.
TODO: make more granular? We only care that each one reader sees consistent boundaries.
      So, we could shallow-copy the stripes list, then have individual locks inside each.
llap-server/src/java/org/apache/hadoop/hive/llap/cache/SerDeLowLevelCacheImpl.java