Index: src/images.py
===================================================================
--- src/images.py	(Revision 51251)
+++ src/images.py	(Arbeitskopie)
@@ -98,6 +98,7 @@
 UCC_IMAGE_INDEX_FILE = 'image-index.txt'
 UCC_IMAGE_INDEX_URL = '%s/%s' % (UCC_BASE_URL, UCC_IMAGE_INDEX_FILE)
 DEFAULT_CHUNK_SIZE = 2**13
+BLOCK_SIZE = 4096  # block size is dependent on file system block size for sparse file
 
 
 def _dummy_progress(*args):
@@ -165,10 +166,23 @@
 					break
 
 				uncompressed_data = decompressor.decompress(compressed_data)
-				fout.write(uncompressed_data)
+
+				# don't write blocks that consist only of 0-bytes to keep sparse file format
+				for i in xrange(0, len(uncompressed_data), BLOCK_SIZE):
+					if len(uncompressed_data[i:i+BLOCK_SIZE]) == BLOCK_SIZE:
+						if uncompressed_data[i:i+BLOCK_SIZE].count('\x00') == BLOCK_SIZE:
+							fout.seek(BLOCK_SIZE, os.SEEK_CUR)
+						else:
+							fout.write(uncompressed_data[i:i+BLOCK_SIZE])
+					# decompressed data can not be always a multiple of BLOCK_SIZE
+					# so write rest data to avoid data loss
+					else:
+						if uncompressed_data[i:].count('\x00') == len(uncompressed_data[i:]):
+							fout.seek(len(uncompressed_data[i:]), os.SEEK_CUR)
+						else:
+							fout.write(uncompressed_data[i:])
+
 				progress(fin.tell(), total_size)
-				del compressed_data
-				del uncompressed_data
 
 				# check free size on disk every N rounds
 				if counter % 50 == 0: