KVM: use cache=none for shared disk templates

Disable host cache for externally mirrored disks to avoid cache incoherency.
Without this, migrations between the same two nodes may end up in disk
corruption.

This is a runtime override of cluster defaults, mostly a workaround.
Unfortunately, we cannot use cmdlib._CheckHVParams, as we want to check the
hypervisor parameters *in conjunction* with the disk template, which is part of
the instance definition.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>

diff --git a/lib/hypervisor/hv_kvm.py b/lib/hypervisor/hv_kvm.py
index 3e9e025..aef5fe1 100644 (file)
--- a/lib/hypervisor/hv_kvm.py
+++ b/lib/hypervisor/hv_kvm.py
@@ -1,7 +1,7 @@
 #
 #
 
-# Copyright (C) 2008, 2009, 2010 Google Inc.
+# Copyright (C) 2008, 2009, 2010, 2011 Google Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -534,7 +534,14 @@ class KVMHypervisor(hv_base.BaseHypervisor):
       if_val = ',if=%s' % disk_type
     # Cache mode
     disk_cache = hvp[constants.HV_DISK_CACHE]
-    if disk_cache != constants.HT_CACHE_DEFAULT:
+    if instance.disk_template in constants.DTS_EXT_MIRROR:
+      if disk_cache != "none":
+        # TODO: make this a hard error, instead of a silent overwrite
+        logging.warning("KVM: overriding disk_cache setting '%s' with 'none'"
+                        " to prevent shared storage corruption on migration",
+                        disk_cache)
+      cache_val = ",cache=none"
+    elif disk_cache != constants.HT_CACHE_DEFAULT:
       cache_val = ",cache=%s" % disk_cache
     else:
       cache_val = ""