code: public boolean reserveSpaceWithCheckSum(Path f, long size) {
      RawInMemoryFileSystem mfs = (RawInMemoryFileSystem)getRawFileSystem();
      synchronized(mfs) {
        boolean b = mfs.reserveSpace(f, size);
        if (b) {
          long checksumSize = getChecksumFileLength(f, size);
          b = mfs.reserveSpace(getChecksumFile(f), checksumSize);
          if (!b) {
            mfs.unreserveSpace(f);
          }
        }
        return b;
      }
    }
docs: Register a file with its size. This will also register a checksum for the
  file that the user is trying to create. This is required since none of
  the FileSystem APIs accept the size of the file as argument. But since it
  is required for us to apriori know the size of the file we are going to
  create, the user must call this method for each file he wants to create
  and reserve memory for that file. We either succeed in reserving memory
  for both the main file and the checksum file and return true, or return
  false.