Use a hint to specify DirectIO to HybridDirectory

server/src/main/java/org/elasticsearch/index/IndexVersions.java

@@ -163,7 +163,7 @@ private static Version parseUnchecked(String version) {
     public static final IndexVersion UPGRADE_TO_LUCENE_10_2_1 = def(9_023_00_0, Version.LUCENE_10_2_1);
     public static final IndexVersion DEFAULT_OVERSAMPLE_VALUE_FOR_BBQ = def(9_024_0_00, Version.LUCENE_10_2_1);
     public static final IndexVersion SEMANTIC_TEXT_DEFAULTS_TO_BBQ = def(9_025_0_00, Version.LUCENE_10_2_1);
-    
+
     public static final IndexVersion UPGRADE_TO_LUCENE_10_3_0 = def(9_050_00_0, Version.LUCENE_10_3_0);
     /*
      * STOP! READ THIS FIRST! No, really,

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/DirectIOIndexInputSupplier.java → server/src/main/java/org/elasticsearch/index/codec/vectors/es818/DirectIOHint.java

@@ -10,14 +10,7 @@
 package org.elasticsearch.index.codec.vectors.es818;
 
 import org.apache.lucene.store.IOContext;
-import org.apache.lucene.store.IndexInput;
 
-import java.io.IOException;
-
-/**
- * A hook for {@link DirectIOLucene99FlatVectorsReader} to specify the input should be opened using DirectIO.
- * Remove when IOContext allows more extensible payloads to be specified.
- */
-public interface DirectIOIndexInputSupplier {
-    IndexInput openInputDirect(String name, IOContext context) throws IOException;
+public enum DirectIOHint implements IOContext.FileOpenHint {
+    INSTANCE
 }

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/DirectIOLucene99FlatVectorsFormat.java

@@ -27,8 +27,14 @@
 import org.apache.lucene.codecs.lucene99.Lucene99FlatVectorsWriter;
 import org.apache.lucene.index.SegmentReadState;
 import org.apache.lucene.index.SegmentWriteState;
+import org.apache.lucene.store.FlushInfo;
+import org.apache.lucene.store.IOContext;
+import org.apache.lucene.store.MergeInfo;
+import org.apache.lucene.store.ReadAdvice;
 
 import java.io.IOException;
+import java.util.Optional;
+import java.util.Set;
 
 /**
  * Copied from Lucene99FlatVectorsFormat in Lucene 10.1
@@ -61,22 +67,62 @@ public FlatVectorsWriter fieldsWriter(SegmentWriteState state) throws IOExceptio
         return new Lucene99FlatVectorsWriter(state, vectorsScorer);
     }
 
+    private static final IOContext DIRECT_IO_CONTEXT = new IOContext() {
+        @Override
+        public Context context() {
+            return Context.DEFAULT;
+        }
+
+        @Override
+        public MergeInfo mergeInfo() {
+            return null;
+        }
+
+        @Override
+        public FlushInfo flushInfo() {
+            return null;
+        }
+
+        @Override
+        public Set<FileOpenHint> hints() {
+            return Set.of(DirectIOHint.INSTANCE);
+        }
+
+        @Override
+        public IOContext withHints(FileOpenHint... hints) {
+            return this;
+        }
+
+        @Override
+        public Optional<ReadAdvice> readAdvice() {
+            return Optional.empty();
+        }
+
+        @Override
+        public IOContext withReadAdvice(ReadAdvice advice) {
+            return this;
+        }
+    };
+
     @Override
     public FlatVectorsReader fieldsReader(SegmentReadState state) throws IOException {
-        if (DirectIOLucene99FlatVectorsReader.shouldUseDirectIO(state)) {
-            // Use mmap for merges and direct I/O for searches.
-            // TODO: Open the mmap file with sequential access instead of random (current behavior).
-            return new MergeReaderWrapper(
-                new DirectIOLucene99FlatVectorsReader(state, vectorsScorer),
-                new Lucene99FlatVectorsReader(state, vectorsScorer)
-            );
-        } else {
-            return new Lucene99FlatVectorsReader(state, vectorsScorer);
-        }
+        SegmentReadState directIOState = new SegmentReadState(
+            state.directory,
+            state.segmentInfo,
+            state.fieldInfos,
+            DIRECT_IO_CONTEXT,
+            state.segmentSuffix
+        );
+        // Use mmap for merges and direct I/O for searches.
+        // TODO: Open the mmap file with sequential access instead of random (current behavior).
+        return new MergeReaderWrapper(
+            new Lucene99FlatVectorsReader(directIOState, vectorsScorer),
+            new Lucene99FlatVectorsReader(state, vectorsScorer)
+        );
     }
 
     @Override
     public String toString() {
-        return "ES818FlatVectorsFormat(" + "vectorsScorer=" + vectorsScorer + ')';
+        return "Lucene99FlatVectorsFormat(" + "vectorsScorer=" + vectorsScorer + ')';
     }
 }