Log NOT_PREFERRED shard movements

server/src/main/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocator.java

@@ -77,6 +77,7 @@
 public class BalancedShardsAllocator implements ShardsAllocator {
 
     private static final Logger logger = LogManager.getLogger(BalancedShardsAllocator.class);
+    private static final Logger notPreferredLogger = LogManager.getLogger(BalancedShardsAllocator.class.getName() + ".not-preferred");
 
     public static final Setting<Float> SHARD_BALANCE_FACTOR_SETTING = Setting.floatSetting(
         "cluster.routing.allocation.balance.shard",
@@ -866,6 +867,14 @@ public boolean moveShards() {
                 // can use the cached decision.
                 final var moveDecision = shardMoved ? decideMove(index, shardRouting) : storedShardMovement.moveDecision();
                 if (moveDecision.isDecisionTaken() && moveDecision.cannotRemainAndCanMove()) {
+                    if (notPreferredLogger.isDebugEnabled()) {
+                        notPreferredLogger.debug(
+                            "Moving shard [{}] to [{}] from a NOT_PREFERRED allocation, explanation is [{}]",
+                            shardRouting,
+                            moveDecision.getTargetNode().getName(),
+                            moveDecision.getCanRemainDecision().getExplanation()
+                        );
+                    }
                     executeMove(shardRouting, index, moveDecision, "move-non-preferred");
                     // Return after a single move so that the change can be simulated before further moves are made.
                     return true;

server/src/main/java/org/elasticsearch/cluster/routing/allocation/decider/WriteLoadConstraintDecider.java

@@ -20,6 +20,7 @@
 import org.elasticsearch.cluster.routing.allocation.WriteLoadConstraintSettings;
 import org.elasticsearch.common.FrequencyCappedAction;
 import org.elasticsearch.common.settings.ClusterSettings;
+import org.elasticsearch.core.Nullable;
 import org.elasticsearch.core.Strings;
 import org.elasticsearch.core.TimeValue;
 import org.elasticsearch.threadpool.ThreadPool;
@@ -154,14 +155,16 @@ public Decision canRemain(IndexMetadata indexMetadata, ShardRouting shardRouting
         var nodeWriteThreadPoolQueueLatencyThreshold = writeLoadConstraintSettings.getQueueLatencyThreshold();
         if (nodeWriteThreadPoolStats.maxThreadPoolQueueLatencyMillis() >= nodeWriteThreadPoolQueueLatencyThreshold.millis()) {
             if (logger.isDebugEnabled() || allocation.debugDecision()) {
+                final Double shardWriteLoad = getShardWriteLoad(allocation, shardRouting);
                 final String explain = Strings.format(
                     """
                         Node [%s] has a queue latency of [%d] millis that exceeds the queue latency threshold of [%s]. This node is \
-                        hot-spotting. Current thread pool utilization [%f]. Moving shard(s) away.""",
+                        hot-spotting. Current thread pool utilization [%f]. Shard write load [%s]. Moving shard(s) away.""",
                     node.nodeId(),
                     nodeWriteThreadPoolStats.maxThreadPoolQueueLatencyMillis(),
                     nodeWriteThreadPoolQueueLatencyThreshold.toHumanReadableString(2),
-                    nodeWriteThreadPoolStats.averageThreadPoolUtilization()
+                    nodeWriteThreadPoolStats.averageThreadPoolUtilization(),
+                    shardWriteLoad == null ? "unknown" : shardWriteLoad
                 );
                 if (logger.isDebugEnabled()) {
                     logCanRemainMessage.maybeExecute(() -> logger.debug(explain));
@@ -182,6 +185,11 @@ public Decision canRemain(IndexMetadata indexMetadata, ShardRouting shardRouting
         );
     }
 
+    @Nullable
+    private Double getShardWriteLoad(RoutingAllocation allocation, ShardRouting shardRouting) {
+        return allocation.clusterInfo().getShardWriteLoads().get(shardRouting.shardId());
+    }
+
     /**
      * Calculates the change to the node's write thread pool utilization percentage if the shard is added to the node.
      * Returns the percent thread pool utilization change.

server/src/test/java/org/elasticsearch/cluster/routing/allocation/allocator/BalancedShardsAllocatorTests.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.cluster.routing.allocation.allocator;
 
+import org.apache.logging.log4j.Level;
 import org.elasticsearch.action.ActionListener;
 import org.elasticsearch.action.support.replication.ClusterStateCreationUtils;
 import org.elasticsearch.cluster.ClusterInfo;
@@ -55,7 +56,9 @@
 import org.elasticsearch.index.IndexVersion;
 import org.elasticsearch.index.shard.ShardId;
 import org.elasticsearch.snapshots.SnapshotShardSizeInfo;
+import org.elasticsearch.test.MockLog;
 import org.elasticsearch.test.gateway.TestGatewayAllocator;
+import org.elasticsearch.test.junit.annotations.TestLogging;
 import org.hamcrest.Matchers;
 
 import java.util.ArrayList;
@@ -1004,6 +1007,43 @@ public void testReturnEarlyOnShardAssignmentChanges() {
         applyStartedShardsUntilNoChange(clusterState, allocationService);
     }
 
+    @TestLogging(
+        value = "org.elasticsearch.cluster.routing.allocation.allocator.BalancedShardsAllocator.not-preferred:DEBUG",
+        reason = "debug logging for test"
+    )
+    public void testNotPreferredMovementIsLoggedAtDebugLevel() {
+        final var clusterState = ClusterStateCreationUtils.state(randomIdentifier(), 3, 3);
+        final var balancedShardsAllocator = new BalancedShardsAllocator(
+            BalancerSettings.DEFAULT,
+            TEST_WRITE_LOAD_FORECASTER,
+            new GlobalBalancingWeightsFactory(BalancerSettings.DEFAULT)
+        );
+
+        final var allocation = new RoutingAllocation(new AllocationDeciders(List.<AllocationDecider>of(new AllocationDecider() {
+            @Override
+            public Decision canRemain(
+                IndexMetadata indexMetadata,
+                ShardRouting shardRouting,
+                RoutingNode node,
+                RoutingAllocation allocation
+            ) {
+                return new Decision.Single(Decision.Type.NOT_PREFERRED, "test_decider", "Always NOT_PREFERRED");
+            }
+        })), clusterState.getRoutingNodes().mutableCopy(), clusterState, ClusterInfo.EMPTY, SnapshotShardSizeInfo.EMPTY, 0L);
+
+        final var notPreferredLoggerName = BalancedShardsAllocator.class.getName() + ".not-preferred";
+        MockLog.assertThatLogger(
+            () -> balancedShardsAllocator.allocate(allocation),
+            notPreferredLoggerName,
+            new MockLog.SeenEventExpectation(
+                "moved a NOT_PREFERRED allocation",
+                notPreferredLoggerName,
+                Level.DEBUG,
+                "Moving shard [*] to [*] from a NOT_PREFERRED allocation, explanation is [Always NOT_PREFERRED]"
+            )
+        );
+    }
+
     /**
      * Test for {@link PrioritiseByShardWriteLoadComparator}. See Comparator Javadoc for expected
      * ordering.

test/framework/src/main/java/org/elasticsearch/test/MockLog.java

@@ -447,6 +447,13 @@ private static void addToMockLogs(MockLog mockLog, List<String> loggers) {
      * Executes an action and verifies expectations against the provided logger
      */
     public static void assertThatLogger(Runnable action, Class<?> loggerOwner, MockLog.LoggingExpectation... expectations) {
+        assertThatLogger(action, loggerOwner.getCanonicalName(), expectations);
+    }
+
+    /**
+     * Executes an action and verifies expectations against the provided logger
+     */
+    public static void assertThatLogger(Runnable action, String loggerOwner, MockLog.LoggingExpectation... expectations) {
         try (var mockLog = MockLog.capture(loggerOwner)) {
             for (var expectation : expectations) {
                 mockLog.addExpectation(expectation);