Add an integration test to verify DirectIO is used for at least one case

server/src/internalClusterTest/java/org/elasticsearch/index/store/DirectIOIT.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+package org.elasticsearch.index.store;
+
+import org.apache.logging.log4j.Level;
+import org.apache.lucene.tests.util.LuceneTestCase;
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.plugins.Plugin;
+import org.elasticsearch.search.vectors.KnnSearchBuilder;
+import org.elasticsearch.search.vectors.VectorData;
+import org.elasticsearch.test.ESIntegTestCase;
+import org.elasticsearch.test.InternalSettingsPlugin;
+import org.elasticsearch.test.MockLog;
+import org.elasticsearch.test.junit.annotations.TestLogging;
+
+import java.util.Collection;
+import java.util.List;
+import java.util.stream.IntStream;
+
+@LuceneTestCase.SuppressCodecs("*") // only use our own codecs
+public class DirectIOIT extends ESIntegTestCase {
+
+    @Override
+    protected Collection<Class<? extends Plugin>> nodePlugins() {
+        return List.of(InternalSettingsPlugin.class);
+    }
+
+    private void indexVectors() {
+        internalCluster().startNode();
+        prepareCreate("vectors").setSettings(Settings.builder().put(InternalSettingsPlugin.USE_COMPOUND_FILE.getKey(), false))
+            .setMapping("""
+                {
+                  "properties": {
+                    "vector": {
+                      "type": "dense_vector",
+                      "dims": 64,
+                      "element_type": "float",
+                      "index": true,
+                      "similarity": "l2_norm",
+                      "index_options": {
+                        "type": "bbq_flat"
+                      }
+                    }
+                  }
+                }
+                """)
+            .get();
+        ensureGreen("vectors");
+
+        for (int i = 0; i < 1000; i++) {
+            indexDoc("vectors", Integer.toString(i), "vector", IntStream.range(0, 64).mapToDouble(d -> randomFloat()).toArray());
+        }
+    }
+
+    @TestLogging(value = "org.elasticsearch.index.store.FsDirectoryFactory:DEBUG", reason = "to capture trace logging for direct IO")
+    public void testDirectIOUsed() {
+        try (MockLog mockLog = MockLog.capture(FsDirectoryFactory.class)) {
+            // we're just looking for some evidence direct IO is used
+            mockLog.addExpectation(
+                new MockLog.PatternSeenEventExpectation(
+                    "Direct IO used",
+                    FsDirectoryFactory.class.getCanonicalName(),
+                    Level.DEBUG,
+                    "Opening .*\\.vec with direct IO"
+                )
+            );
+
+            indexVectors();
+
+            // do a search
+            prepareSearch("vectors").setKnnSearch(
+                List.of(new KnnSearchBuilder("vector", new VectorData(null, new byte[64]), 10, 20, null, null))
+            ).get();
+
+            mockLog.assertAllExpectationsMatched();
+        }
+    }
+}

server/src/main/java/org/elasticsearch/index/store/FsDirectoryFactory.java

@@ -157,6 +157,7 @@ public IndexInput openInput(String name, IOContext context) throws IOException {
             if (directIODelegate != null && context.hints().contains(DirectIOHint.INSTANCE)) {
                 ensureOpen();
                 ensureCanRead(name);
+                Log.debug("Opening {} with direct IO", name);
                 return directIODelegate.openInput(name, context);
             } else if (useDelegate(name, context)) {
                 // we need to do these checks on the outer directory since the inner doesn't know about pending deletes