diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -10,6 +10,7 @@ import stats
 from util import Progress
 from hashtree import HashTree,hashBlock
 from netnode import BaseConnection,NetNode,FailedConnection,LockedException,IncompatibleException
+from datafile import DataFile
 
 
 class DeniedConnection(Exception): pass
@@ -142,28 +143,27 @@ class Client(NetNode):
 				print("The file is locked. Either (a) there's another pull going on (then wait or kill it), or (b) a previous pull ended prematurely and the file is probably corrupt (then repeat pull with -f for force).")
 				return
 		log.info(blocksToTransfer)
-		dataFile=open(self._filename, mode="rb+")
-		i1=-1
+		dataFile=DataFile.open(self._filename, mode="rb+")
 
 		print(datetime.now(), "receiving data:")
 		progress=Progress(len(blocksToTransfer))
-		for (k,i2) in enumerate(blocksToTransfer):
-			self._outcoming.writeMsg({"command":"req", "index":i2, "dataType":"data"})
-			jsonData,binData=self._incoming.readMsg()
-			assert jsonData["command"]=="send" and jsonData["index"]==i2 and jsonData["dataType"]=="data", jsonData
-
-			if i1+1!=i2:
-				dataFile.seek(i2*HashTree.BLOCK_SIZE)
-			dataFile.write(binData)
 
-			if self._treeFile:
-				self._newLeaves[i2+self._tree.leafStart]=hashBlock(binData)
+		for k in range(0,len(blocksToTransfer),conf.batchSize):
+			indices=blocksToTransfer[k:k+conf.batchSize]
+			self._outcoming.writeMsg({"command":"req", "index":indices, "dataType":"data"})
+			jsonData,binData=self._incoming.readMsg()
+			assert jsonData["command"]=="send" and jsonData["index"]==indices and jsonData["dataType"]=="data", jsonData
+			for (j,i) in enumerate(indices):
+				block=binData[j*HashTree.BLOCK_SIZE:(j+1)*HashTree.BLOCK_SIZE]
+				dataFile.writeAt(i,block)
 
-			log.info("block #{0}: {1}...{2}".format(i2,binData[:5],binData[-5:]))
+				if self._treeFile:
+					self._newLeaves[i+self._tree.leafStart]=hashBlock(block)
 
-			stats.logTransferredBlock()
-			i1=i2
-			progress.p(k)
+				log.info("block #{0}: {1}...{2}".format(i,block[:5],block[-5:]))
+
+				stats.logTransferredBlock()
+				progress.p(k+j)
 		progress.done()
 
 		self._outcoming.writeMsg({"command":"end"})