Turn direct IO for BBQ rescoring off by default (#130014)

Add a changelog entry for direct io option

docs/changelog/125921.yaml

@@ -0,0 +1,18 @@
+pr: 125921
+summary: Allow direct IO for BBQ rescoring
+area: Vector Search
+type: feature
+highlight:
+  title: Allow direct IO for BBQ rescoring
+  body: |-
+    BBQ rescoring performance can be drastically affected by the amount of available
+    off-heap RAM for use by the system page cache. When there is not enough off-heap RAM
+    to fit all the vector data in memory, BBQ search latencies can be affected by as much as 5000x.
+    Specifying the `vector.rescoring.directio=true` Java option on all vector search
+    nodes modifies rescoring to use direct IO, which eliminates these very high latencies
+    from searches in low-memory scenarios, at a cost of a reduction
+    in vector search performance for BBQ indices when the vectors do all fit in memory.
+
+    This option is released in 9.1 as a tech preview whilst we analyse its effect
+    for a variety of use cases.
+issues: []

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

@@ -17,6 +17,7 @@ import org.apache.lucene.store.IOContext;
 import org.apache.lucene.store.IndexOutput;
 import org.apache.lucene.tests.util.LuceneTestCase;
 import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.index.codec.vectors.es818.ES818BinaryQuantizedVectorsFormat;
 import org.elasticsearch.plugins.Plugin;
 import org.elasticsearch.search.vectors.KnnSearchBuilder;
 import org.elasticsearch.search.vectors.VectorData;
@@ -43,7 +44,9 @@ import static org.hamcrest.Matchers.is;
 public class DirectIOIT extends ESIntegTestCase {
 
     @BeforeClass
-    public static void checkSupported() throws IOException {
+    public static void checkSupported() {
+        assumeTrue("Direct IO is not enabled", ES818BinaryQuantizedVectorsFormat.USE_DIRECT_IO);
+
         Path path = createTempDir("directIOProbe");
         try (Directory dir = open(path); IndexOutput out = dir.createOutput("out", IOContext.DEFAULT)) {
             out.writeString("test");

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsFormat.java

@@ -87,7 +87,7 @@ import static org.elasticsearch.index.mapper.vectors.DenseVectorFieldMapper.MAX_
  */
 public class ES818BinaryQuantizedVectorsFormat extends FlatVectorsFormat {
 
-    static final boolean USE_DIRECT_IO = Boolean.parseBoolean(System.getProperty("vector.rescoring.directio", "true"));
+    public static final boolean USE_DIRECT_IO = Boolean.parseBoolean(System.getProperty("vector.rescoring.directio", "false"));
 
     public static final String BINARIZED_VECTOR_COMPONENT = "BVEC";
     public static final String NAME = "ES818BinaryQuantizedVectorsFormat";

server/src/test/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsFormatTests.java

@@ -333,6 +333,8 @@ public class ES818BinaryQuantizedVectorsFormatTests extends BaseKnnVectorsFormat
     }
 
     static void checkDirectIOSupported() {
+        assumeTrue("Direct IO is not enabled", ES818BinaryQuantizedVectorsFormat.USE_DIRECT_IO);
+
         Path path = createTempDir("directIOProbe");
         try (Directory dir = open(path); IndexOutput out = dir.createOutput("out", IOContext.DEFAULT)) {
             out.writeString("test");

server/src/test/java/org/elasticsearch/index/codec/vectors/es818/ES818HnswBinaryQuantizedVectorsFormatTests.java

@@ -221,6 +221,8 @@ public class ES818HnswBinaryQuantizedVectorsFormatTests extends BaseKnnVectorsFo
     }
 
     static void checkDirectIOSupported() {
+        assumeTrue("Direct IO is not enabled", ES818BinaryQuantizedVectorsFormat.USE_DIRECT_IO);
+
         Path path = createTempDir("directIOProbe");
         try (Directory dir = open(path); IndexOutput out = dir.createOutput("out", IOContext.DEFAULT)) {
             out.writeString("test");