private static Long getFileSizeForDirName(String fileSizeDirName) {
    Long sizeInBytes = null;
    if (fileSizeDirName.startsWith(FILE_SIZE_DIR_PREFIX)) {
      String sizeString = fileSizeDirName.substring(FILE_SIZE_DIR_PREFIX.length());
      sizeInBytes = Long.parseLong(sizeString);
    }
    return sizeInBytes;
  }
Parse the file size from a file size directory name.
@param fileSizeDirName the name of the file size directory.
@return the parsed size in bytes, or {@code null} if the directory name did not start with the correct prefix.
@throws NumberFormatException if a valid long does not follow the filename prefix.
ambry-store/src/main/java/com/github/ambry/store/DiskSpaceAllocator.java