Fiemap: introduce 'buf_size' parameter for __init__

We are going to use full power of FIEMAP. It requires an array where it will
place the block map on output. Generally, big arrays are good because fiemap
will be called less times, which is good for performance.

Make the default to be a 1MiB buffer, and allocate it in the constructur
for further use.

Change-Id: I4e271794bf747cc7d611d86a6a76afd554c8e09f
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>

diff --git a/bmaptools/Fiemap.py b/bmaptools/Fiemap.py
index f9671d5..d14514e 100644 (file)
--- a/bmaptools/Fiemap.py
+++ b/bmaptools/Fiemap.py
@@ -45,9 +45,14 @@ class Fiemap:
 
         self._f_image_needs_close = True
 
-    def __init__(self, image):
+    def __init__(self, image, buf_size = 1024 * 1024):
         """ Initialize a class instance. The 'image' argument is full path to
-        the file to operate on, or a file object to operate on. """
+        the file to operate on, or a file object to operate on.
+
+        The 'buf_size' argument is the size of the buffer for 'struct
+        fiemap_extent' elements which will be used when invoking the FIEMAP
+        ioctl. The larger is the buffer, the less times the FIEMAP ioctl will
+        be invoked. """
 
         self._f_image_needs_close = False
 
@@ -58,6 +63,21 @@ class Fiemap:
             self._image_path = image
             self._open_image_file()
 
+        # Validate 'buf_size'
+        min_buf_size = _FIEMAP_SIZE + _FIEMAP_EXTENT_SIZE
+        if buf_size < min_buf_size:
+            raise Error("too small buffer (%d bytes), minimum is %d bytes" \
+                        % (buf_size, min_buf_size))
+
+        # How many 'struct fiemap_extent' elements fit the buffer
+        buf_size -= _FIEMAP_SIZE
+        self._fiemap_extent_cnt = buf_size / _FIEMAP_EXTENT_SIZE
+        self._buf_size = self._fiemap_extent_cnt * _FIEMAP_EXTENT_SIZE
+        buf_size += _FIEMAP_SIZE
+
+        # Allocate a mutable buffer for the FIEMAP ioctl
+        self._buf = array.array('B', [0] * self._buf_size)
+
         self.image_size = os.fstat(self._f_image.fileno()).st_size
 
         try: