optimize libvirt image cache usage

When dealing with cow images, minimize the number of downloads,
by caching both the original download and a resized derivative,
with extra disk space being used for the downloaded image.
This part was suggested by Vish Ishaya.

When dealing with raw images, don't bother caching the resized
derivative, as it's quicker to just copy the downloaded image
to the instance dir and resize in place.  Similarly when
generating a raw image, do so directly to the instance dir.

Change-Id: I1458b2c39a51d7d9f5bdfff53155431c863b8a40

1 files changed, 13 insertions, 16 deletions

diff --git a/nova/virt/libvirt/connection.py b/nova/virt/libvirt/connection.py
index 2bd9ac329..eca954b76 100644
--- a/nova/virt/libvirt/connection.py
+++ b/nova/virt/libvirt/connection.py
@@ -828,27 +828,24 @@ class LibvirtConnection(driver.ComputeDriver):
             if cow or not generating:
                 call_if_not_exists(base, fn, *args, **kwargs)
             elif generating:
-                # For raw it's quicker to generate both
-                # FIXME(p-draigbrady) the first call here is probably
-                # redundant, as it's of no benefit to cache in base?
-                call_if_not_exists(base, fn, *args, **kwargs)
-                if os.path.exists(target):
-                    os.unlink(target)
-                fn(target=target, *args, **kwargs)
+                # For raw it's quicker to just generate outside the cache
+                call_if_not_exists(target, fn, *args, **kwargs)
 
             if cow:
+                cow_base = base
                 if size:
-                    disk.extend(base, size)
-                libvirt_utils.create_cow_image(base, target)
+                    size_gb = size / (1024 * 1024 * 1024)
+                    cow_base += "_%d" % size_gb
+                    if not os.path.exists(cow_base):
+                        libvirt_utils.copy_image(base, cow_base)
+                        disk.extend(cow_base, size)
+                libvirt_utils.create_cow_image(cow_base, target)
             elif not generating:
                 libvirt_utils.copy_image(base, target)
                 # Resize after the copy, as it's usually much faster
                 # to make sparse updates, rather than potentially
                 # naively copying the whole image file.
                 if size:
-                    # FIXME(p-draigbrady) the first call here is probably
-                    # redundant, as it's of no benefit have full size in base?
-                    disk.extend(base, size)
                     disk.extend(target, size)
 
     @staticmethod
@@ -944,9 +941,6 @@ class LibvirtConnection(driver.ComputeDriver):
         inst_type = instance_types.get_instance_type(inst_type_id)
         if size == 0 or suffix == '.rescue':
             size = None
-            root_fname += "_sm"
-        else:
-            root_fname += "_%d" % instance['root_gb']
 
         if not self._volume_in_mapping(self.default_root_device,
                                        block_device_info):
@@ -1829,11 +1823,14 @@ class LibvirtConnection(driver.ComputeDriver):
                 backing_file = os.path.join(FLAGS.instances_path,
                                             '_base', info['backing_file'])
 
+                # Remove any size tags which the cache manages
+                cached_file = info['backing_file'].split('_')[0]
+
                 if not os.path.exists(backing_file):
                     self._cache_image(fn=self._fetch_image,
                         context=ctxt,
                         target=info['path'],
-                        fname=info['backing_file'],
+                        fname=cached_file,
                         cow=FLAGS.use_cow_images,
                         image_id=instance_ref['image_ref'],
                         user_id=instance_ref['user_id'],