public static String generateFileSizeDirName(long sizeInBytes) {
    return FILE_SIZE_DIR_PREFIX + sizeInBytes;
  }
@param sizeInBytes the size of the files in this directory
@return a directory name for this size. This is reserve_size_{n} where {n} is {@code sizeInBytes}
ambry-store/src/main/java/com/github/ambry/store/DiskSpaceAllocator.java