[PATCH cluster v2 01/14] datacenter config: add HA auto-rebalance margin mode and cooldown

[Michael Ryom <Michael@RyomHerold.dk>]

The relative improvement margin of the automatic resource balancing
provides almost no hysteresis when a single resource dominates the
cluster load: with two nodes, a resource load v and a node base load
difference d, the relative improvement of moving the resource is
2d / (v + d), so background fluctuations of the node base loads are
enough to qualify a migration and, afterwards, its reverse (see bug
#8059).

Add a margin mode, which allows to interpret the existing margin as a
minimum absolute improvement in percentage points instead, which creates
a dead band for such migrations. It defaults to 'relative', i.e. the
behavior of existing setups does not change.

Also add a cooldown, which keeps resources moved by a resource
balancing migration from being considered for another one for the given
time. It defaults to 0, i.e. it is disabled.

Signed-off-by: Michael Ryom <Michael@RyomHerold.dk>
---
 src/PVE/DataCenterConfig.pm | 28 ++++++++++++++++++++++++++--
 1 file changed, 26 insertions(+), 2 deletions(-)

diff --git a/src/PVE/DataCenterConfig.pm b/src/PVE/DataCenterConfig.pm
index 004122e..e637e63 100644
--- a/src/PVE/DataCenterConfig.pm
+++ b/src/PVE/DataCenterConfig.pm
@@ -75,8 +75,32 @@ EODESC
         maximum => 100,
         default => 10,
         requires => 'ha-auto-rebalance',
-        description => "The minimum relative improvement in cluster node imbalance,"
-            . " in percent, to commit to a resource balancing migration.",
+        description => "The minimum improvement in cluster node imbalance to commit to a"
+            . " resource balancing migration. Depending on the margin mode, either in"
+            . " percent of the current imbalance (relative) or in percentage points"
+            . " (absolute).",
+    },
+    'ha-auto-rebalance-margin-mode' => {
+        type => 'string',
+        enum => ['relative', 'absolute'],
+        optional => 1,
+        default => 'relative',
+        requires => 'ha-auto-rebalance',
+        description => "Whether the margin is the minimum relative or absolute improvement"
+            . " in cluster node imbalance. An absolute margin prevents migrations which"
+            . " only shuffle load without meaningfully improving the cluster balance, e.g."
+            . " a single dominating resource being moved back and forth because of"
+            . " fluctuations of the node base loads.",
+    },
+    'ha-auto-rebalance-cooldown' => {
+        type => 'integer',
+        optional => 1,
+        minimum => 0,
+        default => 0,
+        requires => 'ha-auto-rebalance',
+        description => "The time, in seconds, after a successful resource balancing"
+            . " migration in which the moved resources are not considered for another"
+            . " resource balancing migration. Set to 0 to disable the cooldown.",
     },
 };
 
-- 
2.47.3