code: private long getAvailableBytes(BlockWorkerInfo workerInfo) {
      long mCapacityBytes = workerInfo.getCapacityBytes();
      long mUsedBytes = workerInfo.getUsedBytes();
      return mCapacityBytes - mUsedBytes - mBlockCapacityReserved;
    }
docs: The information of BlockWorkerInfo is update after a file complete write. To avoid evict,
  user should configure "alluxio.user.file.write.avoid.eviction.policy.reserved.size.bytes"
  to reserve some space to store the block.
  
  @param workerInfo BlockWorkerInfo of the worker
  @return the available bytes of the worker