code: int spillInMemoryPartition(Channel.ID targetChannel, IOManager ioManager, LinkedBlockingQueue<MemorySegment> writeBehindBuffers) throws IOException {
  		this.initialPartitionBuffersCount = partitionBuffers.length; // for ReOpenableHashMap
  		this.initialBuildSideChannel = targetChannel;
  		
  		initialBuildSideWriter = ioManager.createBlockChannelWriter(targetChannel, writeBehindBuffers);
  		
  		final int numSegments = this.partitionBuffers.length;
  		for (int i = 0; i < numSegments; i++) {
  			initialBuildSideWriter.writeBlock(partitionBuffers[i]);
  		}
  		this.partitionBuffers = null;
  		initialBuildSideWriter.close();
  		// num partitions are now in the writeBehindBuffers. We propagate this information back
  		return numSegments;
  		
  	}
docs: Spills this partition to disk. This method is invoked once after the initial open() method
  
  @return Number of memorySegments in the writeBehindBuffers!