code: public void updateUsedBytes(String tierAlias, long usedBytesOnTier) {
      mUsedBytes += usedBytesOnTier - mUsedBytesOnTiers.get(tierAlias);
      mUsedBytesOnTiers.put(tierAlias, usedBytesOnTier);
    }
docs: Sets the used space of the worker in bytes.
  
  @param tierAlias alias of storage tier
  @param usedBytesOnTier used bytes on certain storage tier