Adding ability to specify the libvirt cache mode for disk devices

In past versions of Nova it was possible to explicitly configure
the cache mode of disks via the libvirt XML template. The current approach
makes this a derived setting of either “none” or “writethrough” based
on the support of O_DIRECT. Whilst this provides a good set of default
settings it removes the ability of the cloud provider to use other
modes such as “writeback” and “unsafe” which are valuable in certain
configurations.

This change allows the cache mode to be specified on a per-disk type
basis. If a disk type does not have a cache mode specified then the
default behaviour remains unchanged.

DocImpact

bug 1086386

Change-Id: I3d296fe0b4b9b976db02db90ad69fd299cd2096a

nova/tests/test_libvirt.py

@@ -269,6 +269,17 @@ class FakeVolumeDriver(object):
         return ""
 
 
+class FakeConfigGuestDisk(object):
+    def __init__(self, *args, **kwargs):
+        self.source_type = None
+        self.driver_cache = None
+
+
+class FakeConfigGuest(object):
+    def __init__(self, *args, **kwargs):
+        self.driver_cache = None
+
+
 class LibvirtConnTestCase(test.TestCase):
 
     def setUp(self):
@@ -3431,6 +3442,33 @@ class LibvirtConnTestCase(test.TestCase):
         self.assertEqual(got_events[0].transition,
                          virtevent.EVENT_LIFECYCLE_STOPPED)
 
+    def test_set_cache_mode(self):
+        self.flags(disk_cachemodes=['file=directsync'])
+        conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), True)
+        fake_conf = FakeConfigGuestDisk()
+
+        fake_conf.source_type = 'file'
+        conn.set_cache_mode(fake_conf)
+        self.assertEqual(fake_conf.driver_cache, 'directsync')
+
+    def test_set_cache_mode_invalid_mode(self):
+        self.flags(disk_cachemodes=['file=FAKE'])
+        conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), True)
+        fake_conf = FakeConfigGuestDisk()
+
+        fake_conf.source_type = 'file'
+        conn.set_cache_mode(fake_conf)
+        self.assertEqual(fake_conf.driver_cache, None)
+
+    def test_set_cache_mode_invalid_object(self):
+        self.flags(disk_cachemodes=['file=directsync'])
+        conn = libvirt_driver.LibvirtDriver(fake.FakeVirtAPI(), True)
+        fake_conf = FakeConfigGuest()
+
+        fake_conf.driver_cache = 'fake'
+        conn.set_cache_mode(fake_conf)
+        self.assertEqual(fake_conf.driver_cache, 'fake')
+
 
 class HostStateTestCase(test.TestCase):