dm cache: switch the "default" cache replacement policy from mq to smq

The Stochastic multiqueue (SMQ) policy (vs MQ) offers the promise of
less memory utilization, improved performance and increased adaptability
in the face of changing workloads.  SMQ also does not have any
cumbersome tuning knobs.

Users may switch from "mq" to "smq" simply by appropriately reloading a
DM table that is using the cache target.  Doing so will cause all of the
mq policy's hints to be dropped.  Also, performance of the cache may
degrade slightly until smq recalculates the origin device's hotspots
that should be cached.

In the future the "mq" policy will just silently make use of "smq" and
the mq code will be removed.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Joe Thornber <ejt@redhat.com>

1 files changed, 17 insertions, 0 deletions

diff --git a/drivers/md/dm-cache-policy-smq.c b/drivers/md/dm-cache-policy-smq.c
index 66feb307e697..80f02d3330e2 100644
--- a/drivers/md/dm-cache-policy-smq.c
+++ b/drivers/md/dm-cache-policy-smq.c
@@ -1748,6 +1748,15 @@ static struct dm_cache_policy_type smq_policy_type = {
         .create = smq_create
 };
 
+static struct dm_cache_policy_type default_policy_type = {
+        .name = "default",
+        .version = {1, 0, 0},
+        .hint_size = 4,
+        .owner = THIS_MODULE,
+        .create = smq_create,
+        .real = &smq_policy_type
+};
+
 static int __init smq_init(void)
 {
         int r;
@@ -1758,12 +1767,20 @@ static int __init smq_init(void)
                 return -ENOMEM;
         }
 
+        r = dm_cache_policy_register(&default_policy_type);
+        if (r) {
+                DMERR("register failed (as default) %d", r);
+                dm_cache_policy_unregister(&smq_policy_type);
+                return -ENOMEM;
+        }
+
         return 0;
 }
 
 static void __exit smq_exit(void)
 {
         dm_cache_policy_unregister(&smq_policy_type);
+        dm_cache_policy_unregister(&default_policy_type);
 }
 
 module_init(smq_init);