feat: compress image on-the-fly

When copying an uncompressed image, we lightly compress the image on the
fly with zstd on transfer. By that, subsequent zeros (holes) are
automatically collapsed and not transferred, which can give a huge
speedup on uncompressed images. Another advantage is, that zstandard can
be decompressed with a very low overhead (e.g. compared to gz or xz) and
is better suited for low-memory devices.

closes: #20

Signed-off-by: Felix Moessbauer <[email protected]>

mtda/client.py

@@ -17,6 +17,7 @@
 import tempfile
 import time
 import zerorpc
+import zstandard as zstd
 
 from mtda.main import MultiTenantDeviceAccess
 import mtda.constants as CONSTS
@@ -468,9 +469,9 @@ def prepare(self, output_size=None, compression=None):
         compr = self.compression() if compression is None else compression
         self._inputsize = self.size()
         self._outputsize = output_size
-        if output_size is None:
-            if compr == CONSTS.IMAGE.RAW.value:
-                self._outputsize = self._inputsize
+        # if image is uncompressed, we compress on the fly
+        if compr == CONSTS.IMAGE.RAW.value:
+            compr = CONSTS.IMAGE.ZST.value
         self._agent.storage_compression(compr, self._session)
         self._lastreport = time.time()
         self._totalread = 0
@@ -509,11 +510,18 @@ def copy(self):
             raise IOError(f'{self._path}: image not found!')
 
         image = open(self._path, 'rb')
+        comp_on_the_fly = False
+        if self.compression() == CONSTS.IMAGE.RAW.value:
+            cctx = zstd.ZstdCompressor(level=1)
+            comp_on_the_fly = True
+            inputstream = cctx.stream_reader(image)
+        else:
+            inputstream = image
+
         try:
-            data = image.read(self._blksz)
-            dataread = len(data)
-            while self._totalread < self._inputsize:
-                self._totalread += dataread
+            data = inputstream.read(self._blksz)
+            while data:
+                self._totalread = image.tell()
                 self.progress()
 
                 # Write block to shared storage device
@@ -525,16 +533,16 @@ def copy(self):
                                   'shared storage')
                 elif bytes_wanted > 0:
                     # Read next block
-                    data = image.read(bytes_wanted)
-                    dataread = len(data)
+                    data = inputstream.read(bytes_wanted)
                 else:
                     # Agent may continue without further data
                     data = b''
-                    dataread = 0
 
         finally:
-            # Close the local image
-            image.close()
+            if comp_on_the_fly:
+                inputstream.close()
+            else:
+                image.close()
 
     def size(self):
         st = os.stat(self._path)