diff --git a/core/src/main/java/org/elasticsearch/gateway/ReplicaShardAllocator.java b/core/src/main/java/org/elasticsearch/gateway/ReplicaShardAllocator.java
index e2b6f0d27ed..c94e1370c01 100644
--- a/core/src/main/java/org/elasticsearch/gateway/ReplicaShardAllocator.java
+++ b/core/src/main/java/org/elasticsearch/gateway/ReplicaShardAllocator.java
@@ -104,6 +104,8 @@ public abstract class ReplicaShardAllocator extends AbstractComponent {
                             && matchingNodes.isNodeMatchBySyncID(nodeWithHighestMatch) == true) {
                         // we found a better match that has a full sync id match, the existing allocation is not fully synced
                         // so we found a better one, cancel this one
+                        logger.debug("cancelling allocation of replica on [{}], sync id match found on node [{}]",
+                                currentNode, nodeWithHighestMatch);
                         it.moveToUnassigned(new UnassignedInfo(UnassignedInfo.Reason.REALLOCATED_REPLICA,
                                 "existing allocation of replica to [" + currentNode + "] cancelled, sync id match found on node [" + nodeWithHighestMatch + "]",
                                 null, allocation.getCurrentNanoTime(), System.currentTimeMillis()));