diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -105,8 +105,7 @@ class Client(NetNode):
 
 	def sendData(self,blocksToTransfer):
 		log.info(blocksToTransfer)
-		dataFile=open(self._filename, mode="rb")
-		i1=-1
+		dataFile=DataFile.open(self._filename)
 
 		print(datetime.now(), "sending data:")
 		progress=Progress(len(blocksToTransfer))
@@ -116,16 +115,13 @@ class Client(NetNode):
 			blocks=[]
 			for j in range(conf.batchSize.data):
 				if k+j>=len(blocksToTransfer): break
-				i2=blocksToTransfer[k+j]
-				if i1+1!=i2:
-					dataFile.seek(i2*HashTree.BLOCK_SIZE)
-				block=dataFile.read(HashTree.BLOCK_SIZE)
+				i=blocksToTransfer[k+j]
+				block=dataFile.readFrom(i)
 
-				indices.append(i2)
+				indices.append(i)
 				blocks.append(block)
-				log.info("block #{0}: {1}...{2}".format(i2,block[:5],block[-5:]))
+				log.info("block #{0}: {1}...{2}".format(i,block[:5],block[-5:]))
 
-				i1=i2
 				progress.p(k+j)
 			if indices: self._sendData(indices,blocks)
 		progress.done()
diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -7,6 +7,7 @@ from hashtree import hashBlock
 from netnode import BaseConnection,NetNode
 import config as conf
 import status
+from datafile import DataFile
 
 
 class Connection(BaseConnection):
@@ -54,8 +55,7 @@ class Server(NetNode):
 
 		self.BLOCK_SIZE=self._tree.BLOCK_SIZE
 
-		self._lastIndex=-1
-		self._dataFileHandle=None
+		self._dataFileObj=None
 
 	@staticmethod
 	def run(connection,*args):
@@ -65,9 +65,9 @@ class Server(NetNode):
 
 	@property
 	def _dataFile(self):
-		if not self._dataFileHandle:
-			self._dataFileHandle=open(self._filename, mode="rb+")
-		return self._dataFileHandle
+		if not self._dataFileObj:
+			self._dataFileObj=DataFile.open(self._filename, mode="rb+")
+		return self._dataFileObj
 
 	def serve(self):
 		try:
@@ -125,9 +125,7 @@ class Server(NetNode):
 		jsonResponse={"command":"send", "index":index, "dataType":"data"}
 		blocks=[]
 		for i in index:
-			if self._lastIndex+1!=i:
-				self._dataFile.seek(i*self.BLOCK_SIZE)
-			blocks.append(self._dataFile.read(self.BLOCK_SIZE))
+			blocks.append(self._dataFile.readFrom(i))
 
 		return (jsonResponse,b"".join(blocks))
 
@@ -137,11 +135,8 @@ class Server(NetNode):
 
 		indices=jsonData["index"]
 		for (i,k) in enumerate(indices):
-			if self._lastIndex+1!=k:
-				self._dataFile.seek(k*self.BLOCK_SIZE)
 			block=binData[i*self.BLOCK_SIZE:(i+1)*self.BLOCK_SIZE]
-			self._dataFile.write(block)
-			self._lastIndex=k
+			self._dataFile.writeAt(k,block)
 			if self._treeFile:
 				self._newLeaves[k+self._tree.leafStart]=hashBlock(block)
 
@@ -150,7 +145,7 @@ class Server(NetNode):
 	def _finalize(self):
 		log.info("closing session...")
 		self._dataFile.close()
-		self._dataFileHandle=None
+		self._dataFileObj=None
 		if self._treeFile:
 			self._updateTree()
 		log.info("done")