diff --git a/Documentation/config-gerrit.txt b/Documentation/config-gerrit.txt
index 3594626..6d98877 100644
--- a/Documentation/config-gerrit.txt
+++ b/Documentation/config-gerrit.txt
@@ -2748,7 +2748,7 @@
 [[index.batchThreads]]index.batchThreads::
 +
 Number of threads to use for indexing in background operations, such as
-online schema upgrades, and also for offline reindexing.
+online schema upgrades, and also the default for offline reindexing.
 +
 If not set or set to a zero, defaults to the number of logical CPUs as returned
 by the JVM. If set to a negative value, defaults to a direct executor.
@@ -2964,6 +2964,11 @@
 +
 Defaults to true (throttling enabled).
 
+During offline reindexing, setting ramBufferSize greater than the size
+of index (size of specific index folder under <site_dir>/index) and
+maxBufferedDocs as -1 avoids unnecessary flushes and triggers only a
+single flush at the end of the process.
+
 Sample Lucene index configuration:
 ----
 [index]
diff --git a/Documentation/pgm-reindex.txt b/Documentation/pgm-reindex.txt
index 5167277..5392564 100644
--- a/Documentation/pgm-reindex.txt
+++ b/Documentation/pgm-reindex.txt
@@ -20,7 +20,8 @@
 
 == OPTIONS
 --threads::
-	Number of threads to use for indexing.
+	Number of threads to use for indexing. Default is
+	link:config-gerrit.html#index.batchThreads[index.batchThreads]
 
 --changes-schema-version::
 	Schema version to reindex; default is most recent version.
diff --git a/java/com/google/gerrit/acceptance/GerritServer.java b/java/com/google/gerrit/acceptance/GerritServer.java
index 5e8698f..34f514c 100644
--- a/java/com/google/gerrit/acceptance/GerritServer.java
+++ b/java/com/google/gerrit/acceptance/GerritServer.java
@@ -41,6 +41,7 @@
 import com.google.gerrit.server.config.SitePath;
 import com.google.gerrit.server.git.receive.AsyncReceiveCommits;
 import com.google.gerrit.server.schema.JdbcAccountPatchReviewStore;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.ssh.NoSshModule;
 import com.google.gerrit.server.util.SocketUtil;
 import com.google.gerrit.server.util.SystemLog;
@@ -374,7 +375,8 @@
     cfg.setString(
         "accountPatchReviewDb", null, "url", JdbcAccountPatchReviewStore.TEST_IN_MEMORY_URL);
     daemon.setEnableHttpd(desc.httpd());
-    daemon.setLuceneModule(LuceneIndexModule.singleVersionAllLatest(0, isSlave(baseConfig)));
+    daemon.setLuceneModule(
+        LuceneIndexModule.singleVersionAllLatest(0, isSlave(baseConfig), AutoFlush.ENABLED));
     daemon.setDatabaseForTesting(
         ImmutableList.of(
             new InMemoryTestingDatabaseModule(cfg, site, inMemoryRepoManager),
diff --git a/java/com/google/gerrit/httpd/init/WebAppInitializer.java b/java/com/google/gerrit/httpd/init/WebAppInitializer.java
index bf5cd2a..c9dfc58 100644
--- a/java/com/google/gerrit/httpd/init/WebAppInitializer.java
+++ b/java/com/google/gerrit/httpd/init/WebAppInitializer.java
@@ -73,6 +73,7 @@
 import com.google.gerrit.server.git.GitRepositoryManagerModule;
 import com.google.gerrit.server.git.SearchingChangeCacheImpl;
 import com.google.gerrit.server.git.WorkQueue;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.OnlineUpgrader;
@@ -344,7 +345,7 @@
   private Module createIndexModule() {
     switch (indexType) {
       case LUCENE:
-        return LuceneIndexModule.latestVersion(false);
+        return LuceneIndexModule.latestVersion(false, AutoFlush.ENABLED);
       case ELASTICSEARCH:
         return ElasticIndexModule.latestVersion(false);
       default:
diff --git a/java/com/google/gerrit/lucene/AbstractLuceneIndex.java b/java/com/google/gerrit/lucene/AbstractLuceneIndex.java
index 7a0430c..93c0fad 100644
--- a/java/com/google/gerrit/lucene/AbstractLuceneIndex.java
+++ b/java/com/google/gerrit/lucene/AbstractLuceneIndex.java
@@ -43,6 +43,7 @@
 import com.google.gerrit.index.query.ListResultSet;
 import com.google.gerrit.index.query.ResultSet;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.logging.LoggingContextAwareExecutorService;
 import com.google.gerrit.server.logging.LoggingContextAwareScheduledExecutorService;
@@ -101,6 +102,7 @@
   private final ReferenceManager<IndexSearcher> searcherManager;
   private final ControlledRealTimeReopenThread<IndexSearcher> reopenThread;
   private final Set<NrtFuture> notDoneNrtFutures;
+  private final AutoFlush autoFlush;
   private ScheduledExecutorService autoCommitExecutor;
 
   AbstractLuceneIndex(
@@ -110,12 +112,14 @@
       String name,
       String subIndex,
       GerritIndexWriterConfig writerConfig,
-      SearcherFactory searcherFactory)
+      SearcherFactory searcherFactory,
+      AutoFlush autoFlush)
       throws IOException {
     this.schema = schema;
     this.sitePaths = sitePaths;
     this.dir = dir;
     this.name = name;
+    this.autoFlush = autoFlush;
     String index = Joiner.on('_').skipNulls().join(name, subIndex);
     long commitPeriod = writerConfig.getCommitWithinMs();
 
@@ -209,7 +213,9 @@
           }
         });
 
-    reopenThread.start();
+    if (autoFlush.equals(AutoFlush.ENABLED)) {
+      reopenThread.start();
+    }
   }
 
   @Override
@@ -459,6 +465,9 @@
     }
 
     private boolean isGenAvailableNowForCurrentSearcher() {
+      if (autoFlush.equals(AutoFlush.DISABLED)) {
+        return true;
+      }
       try {
         return reopenThread.waitForGeneration(gen, 0);
       } catch (InterruptedException e) {
diff --git a/java/com/google/gerrit/lucene/ChangeSubIndex.java b/java/com/google/gerrit/lucene/ChangeSubIndex.java
index 98424b5..8fcd0b1 100644
--- a/java/com/google/gerrit/lucene/ChangeSubIndex.java
+++ b/java/com/google/gerrit/lucene/ChangeSubIndex.java
@@ -29,6 +29,7 @@
 import com.google.gerrit.index.query.QueryParseException;
 import com.google.gerrit.reviewdb.client.Change;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.change.ChangeField;
 import com.google.gerrit.server.index.change.ChangeIndex;
 import com.google.gerrit.server.query.change.ChangeData;
@@ -48,7 +49,8 @@
       SitePaths sitePaths,
       Path path,
       GerritIndexWriterConfig writerConfig,
-      SearcherFactory searcherFactory)
+      SearcherFactory searcherFactory,
+      AutoFlush autoFlush)
       throws IOException {
     this(
         schema,
@@ -56,7 +58,8 @@
         FSDirectory.open(path),
         path.getFileName().toString(),
         writerConfig,
-        searcherFactory);
+        searcherFactory,
+        autoFlush);
   }
 
   ChangeSubIndex(
@@ -65,9 +68,10 @@
       Directory dir,
       String subIndex,
       GerritIndexWriterConfig writerConfig,
-      SearcherFactory searcherFactory)
+      SearcherFactory searcherFactory,
+      AutoFlush autoFlush)
       throws IOException {
-    super(schema, sitePaths, dir, NAME, subIndex, writerConfig, searcherFactory);
+    super(schema, sitePaths, dir, NAME, subIndex, writerConfig, searcherFactory, autoFlush);
   }
 
   @Override
diff --git a/java/com/google/gerrit/lucene/LuceneAccountIndex.java b/java/com/google/gerrit/lucene/LuceneAccountIndex.java
index 0b787b6..272c774 100644
--- a/java/com/google/gerrit/lucene/LuceneAccountIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneAccountIndex.java
@@ -32,6 +32,7 @@
 import com.google.gerrit.server.account.AccountState;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.index.account.AccountIndex;
 import com.google.inject.Inject;
@@ -87,7 +88,8 @@
       @GerritServerConfig Config cfg,
       SitePaths sitePaths,
       Provider<AccountCache> accountCache,
-      @Assisted Schema<AccountState> schema)
+      @Assisted Schema<AccountState> schema,
+      AutoFlush autoFlush)
       throws IOException {
     super(
         schema,
@@ -96,7 +98,8 @@
         ACCOUNTS,
         null,
         new GerritIndexWriterConfig(cfg, ACCOUNTS),
-        new SearcherFactory());
+        new SearcherFactory(),
+        autoFlush);
     this.accountCache = accountCache;
 
     indexWriterConfig = new GerritIndexWriterConfig(cfg, ACCOUNTS);
diff --git a/java/com/google/gerrit/lucene/LuceneChangeIndex.java b/java/com/google/gerrit/lucene/LuceneChangeIndex.java
index 87f9396..a9e431e 100644
--- a/java/com/google/gerrit/lucene/LuceneChangeIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneChangeIndex.java
@@ -55,6 +55,7 @@
 import com.google.gerrit.server.StarredChangesUtil;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexExecutor;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.index.change.ChangeField;
@@ -155,7 +156,8 @@
       SitePaths sitePaths,
       @IndexExecutor(INTERACTIVE) ListeningExecutorService executor,
       ChangeData.Factory changeDataFactory,
-      @Assisted Schema<ChangeData> schema)
+      @Assisted Schema<ChangeData> schema,
+      AutoFlush autoFlush)
       throws IOException {
     this.executor = executor;
     this.changeDataFactory = changeDataFactory;
@@ -170,18 +172,35 @@
     if (LuceneIndexModule.isInMemoryTest(cfg)) {
       openIndex =
           new ChangeSubIndex(
-              schema, sitePaths, new RAMDirectory(), "ramOpen", openConfig, searcherFactory);
+              schema,
+              sitePaths,
+              new RAMDirectory(),
+              "ramOpen",
+              openConfig,
+              searcherFactory,
+              autoFlush);
       closedIndex =
           new ChangeSubIndex(
-              schema, sitePaths, new RAMDirectory(), "ramClosed", closedConfig, searcherFactory);
+              schema,
+              sitePaths,
+              new RAMDirectory(),
+              "ramClosed",
+              closedConfig,
+              searcherFactory,
+              autoFlush);
     } else {
       Path dir = LuceneVersionManager.getDir(sitePaths, CHANGES, schema);
       openIndex =
           new ChangeSubIndex(
-              schema, sitePaths, dir.resolve(CHANGES_OPEN), openConfig, searcherFactory);
+              schema, sitePaths, dir.resolve(CHANGES_OPEN), openConfig, searcherFactory, autoFlush);
       closedIndex =
           new ChangeSubIndex(
-              schema, sitePaths, dir.resolve(CHANGES_CLOSED), closedConfig, searcherFactory);
+              schema,
+              sitePaths,
+              dir.resolve(CHANGES_CLOSED),
+              closedConfig,
+              searcherFactory,
+              autoFlush);
     }
   }
 
diff --git a/java/com/google/gerrit/lucene/LuceneGroupIndex.java b/java/com/google/gerrit/lucene/LuceneGroupIndex.java
index 0fdef77..47a5038 100644
--- a/java/com/google/gerrit/lucene/LuceneGroupIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneGroupIndex.java
@@ -30,6 +30,7 @@
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
 import com.google.gerrit.server.group.InternalGroup;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.index.group.GroupIndex;
 import com.google.inject.Inject;
@@ -83,7 +84,8 @@
       @GerritServerConfig Config cfg,
       SitePaths sitePaths,
       Provider<GroupCache> groupCache,
-      @Assisted Schema<InternalGroup> schema)
+      @Assisted Schema<InternalGroup> schema,
+      AutoFlush autoFlush)
       throws IOException {
     super(
         schema,
@@ -92,7 +94,8 @@
         GROUPS,
         null,
         new GerritIndexWriterConfig(cfg, GROUPS),
-        new SearcherFactory());
+        new SearcherFactory(),
+        autoFlush);
     this.groupCache = groupCache;
 
     indexWriterConfig = new GerritIndexWriterConfig(cfg, GROUPS);
diff --git a/java/com/google/gerrit/lucene/LuceneIndexModule.java b/java/com/google/gerrit/lucene/LuceneIndexModule.java
index 302a2da..10b58c0 100644
--- a/java/com/google/gerrit/lucene/LuceneIndexModule.java
+++ b/java/com/google/gerrit/lucene/LuceneIndexModule.java
@@ -19,6 +19,7 @@
 import com.google.gerrit.index.project.ProjectIndex;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.index.AbstractIndexModule;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.VersionManager;
 import com.google.gerrit.server.index.account.AccountIndex;
 import com.google.gerrit.server.index.change.ChangeIndex;
@@ -28,25 +29,36 @@
 import org.eclipse.jgit.lib.Config;
 
 public class LuceneIndexModule extends AbstractIndexModule {
-  public static LuceneIndexModule singleVersionAllLatest(int threads, boolean slave) {
-    return new LuceneIndexModule(ImmutableMap.of(), threads, slave);
+  private final AutoFlush autoFlush;
+
+  public static LuceneIndexModule singleVersionAllLatest(
+      int threads, boolean slave, AutoFlush autoFlush) {
+    return new LuceneIndexModule(ImmutableMap.of(), threads, slave, autoFlush);
   }
 
   public static LuceneIndexModule singleVersionWithExplicitVersions(
-      Map<String, Integer> versions, int threads, boolean slave) {
-    return new LuceneIndexModule(versions, threads, slave);
+      Map<String, Integer> versions, int threads, boolean slave, AutoFlush autoFlush) {
+    return new LuceneIndexModule(versions, threads, slave, autoFlush);
   }
 
-  public static LuceneIndexModule latestVersion(boolean slave) {
-    return new LuceneIndexModule(null, 0, slave);
+  public static LuceneIndexModule latestVersion(boolean slave, AutoFlush autoFlush) {
+    return new LuceneIndexModule(null, 0, slave, autoFlush);
   }
 
   static boolean isInMemoryTest(Config cfg) {
     return cfg.getBoolean("index", "lucene", "testInmemory", false);
   }
 
-  private LuceneIndexModule(Map<String, Integer> singleVersions, int threads, boolean slave) {
+  private LuceneIndexModule(
+      Map<String, Integer> singleVersions, int threads, boolean slave, AutoFlush autoFlush) {
     super(singleVersions, threads, slave);
+    this.autoFlush = autoFlush;
+  }
+
+  @Override
+  protected void configure() {
+    super.configure();
+    bind(AutoFlush.class).toInstance(autoFlush);
   }
 
   @Override
diff --git a/java/com/google/gerrit/lucene/LuceneProjectIndex.java b/java/com/google/gerrit/lucene/LuceneProjectIndex.java
index 950e206..601ed5f 100644
--- a/java/com/google/gerrit/lucene/LuceneProjectIndex.java
+++ b/java/com/google/gerrit/lucene/LuceneProjectIndex.java
@@ -30,6 +30,7 @@
 import com.google.gerrit.reviewdb.client.Project;
 import com.google.gerrit.server.config.GerritServerConfig;
 import com.google.gerrit.server.config.SitePaths;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexUtils;
 import com.google.gerrit.server.project.ProjectCache;
 import com.google.gerrit.server.project.ProjectState;
@@ -83,7 +84,8 @@
       @GerritServerConfig Config cfg,
       SitePaths sitePaths,
       Provider<ProjectCache> projectCache,
-      @Assisted Schema<ProjectData> schema)
+      @Assisted Schema<ProjectData> schema,
+      AutoFlush autoFlush)
       throws IOException {
     super(
         schema,
@@ -92,7 +94,8 @@
         PROJECTS,
         null,
         new GerritIndexWriterConfig(cfg, PROJECTS),
-        new SearcherFactory());
+        new SearcherFactory(),
+        autoFlush);
     this.projectCache = projectCache;
 
     indexWriterConfig = new GerritIndexWriterConfig(cfg, PROJECTS);
diff --git a/java/com/google/gerrit/pgm/Daemon.java b/java/com/google/gerrit/pgm/Daemon.java
index 8887385..6b4a065 100644
--- a/java/com/google/gerrit/pgm/Daemon.java
+++ b/java/com/google/gerrit/pgm/Daemon.java
@@ -82,6 +82,7 @@
 import com.google.gerrit.server.git.SearchingChangeCacheImpl;
 import com.google.gerrit.server.git.WorkQueue;
 import com.google.gerrit.server.group.PeriodicGroupIndexer;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.OnlineUpgrader;
@@ -498,7 +499,7 @@
     }
     switch (indexType) {
       case LUCENE:
-        return LuceneIndexModule.latestVersion(slave);
+        return LuceneIndexModule.latestVersion(slave, AutoFlush.ENABLED);
       case ELASTICSEARCH:
         return ElasticIndexModule.latestVersion(slave);
       default:
diff --git a/java/com/google/gerrit/pgm/Reindex.java b/java/com/google/gerrit/pgm/Reindex.java
index 6fd5424..544c61d 100644
--- a/java/com/google/gerrit/pgm/Reindex.java
+++ b/java/com/google/gerrit/pgm/Reindex.java
@@ -30,6 +30,7 @@
 import com.google.gerrit.pgm.util.SiteProgram;
 import com.google.gerrit.server.change.ChangeResource;
 import com.google.gerrit.server.config.GerritServerConfig;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.change.ChangeSchemaDefinitions;
@@ -51,8 +52,10 @@
 import org.kohsuke.args4j.Option;
 
 public class Reindex extends SiteProgram {
-  @Option(name = "--threads", usage = "Number of threads to use for indexing")
-  private int threads = Runtime.getRuntime().availableProcessors();
+  @Option(
+      name = "--threads",
+      usage = "Number of threads to use for indexing. Default is index.batchThreads from config.")
+  private int threads = 0;
 
   @Option(
       name = "--changes-schema-version",
@@ -149,7 +152,9 @@
     Module indexModule;
     switch (IndexModule.getIndexType(dbInjector)) {
       case LUCENE:
-        indexModule = LuceneIndexModule.singleVersionWithExplicitVersions(versions, threads, slave);
+        indexModule =
+            LuceneIndexModule.singleVersionWithExplicitVersions(
+                versions, threads, slave, AutoFlush.DISABLED);
         break;
       case ELASTICSEARCH:
         indexModule =
diff --git a/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java b/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java
index 12a44dc..54618a1 100644
--- a/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java
+++ b/java/com/google/gerrit/pgm/init/index/lucene/LuceneIndexModuleOnInit.java
@@ -17,6 +17,7 @@
 import com.google.gerrit.lucene.LuceneAccountIndex;
 import com.google.gerrit.lucene.LuceneGroupIndex;
 import com.google.gerrit.pgm.init.index.IndexModuleOnInit;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.account.AccountIndex;
 import com.google.gerrit.server.index.group.GroupIndex;
 import com.google.inject.AbstractModule;
@@ -36,5 +37,7 @@
             .build(GroupIndex.Factory.class));
 
     install(new IndexModuleOnInit());
+
+    bind(AutoFlush.class).toInstance(AutoFlush.DISABLED);
   }
 }
diff --git a/java/com/google/gerrit/server/git/meta/VersionedMetaData.java b/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
index 8694299..cce81b0 100644
--- a/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
+++ b/java/com/google/gerrit/server/git/meta/VersionedMetaData.java
@@ -213,6 +213,27 @@
   }
 
   /**
+   * Update this metadata branch, recording a new commit on its reference. This method mutates its
+   * receiver.
+   *
+   * @param update helper information to define the update that will occur.
+   * @param objInserter Shared object inserter.
+   * @param objReader Shared object reader.
+   * @param revWalk Shared rev walk.
+   * @return the commit that was created
+   * @throws IOException if there is a storage problem and the update cannot be executed as
+   *     requested or if it failed because of a concurrent update to the same reference
+   */
+  public RevCommit commit(
+      MetaDataUpdate update, ObjectInserter objInserter, ObjectReader objReader, RevWalk revWalk)
+      throws IOException {
+    try (BatchMetaDataUpdate batch = openUpdate(update, objInserter, objReader, revWalk)) {
+      batch.write(update.getCommitBuilder());
+      return batch.commit();
+    }
+  }
+
+  /**
    * Creates a new commit and a new ref based on this commit. This method mutates its receiver.
    *
    * @param update helper information to define the update that will occur.
@@ -259,11 +280,39 @@
    * @throws IOException if the update failed.
    */
   public BatchMetaDataUpdate openUpdate(MetaDataUpdate update) throws IOException {
+    return openUpdate(update, null, null, null);
+  }
+
+  /**
+   * Open a batch of updates to the same metadata ref.
+   *
+   * <p>This allows making multiple commits to a single metadata ref, at the end of which is a
+   * single ref update. For batching together updates to multiple refs (each consisting of one or
+   * more commits against their respective refs), create the {@link MetaDataUpdate} with a {@link
+   * BatchRefUpdate}.
+   *
+   * <p>A ref update produced by this {@link BatchMetaDataUpdate} is only committed if there is no
+   * associated {@link BatchRefUpdate}. As a result, the configured ref updated event is not fired
+   * if there is an associated batch.
+   *
+   * <p>If object inserter, reader and revwalk are provided, then the updates are not flushed,
+   * allowing callers the flexibility to flush only once after several updates.
+   *
+   * @param update helper info about the update.
+   * @param objInserter Shared object inserter.
+   * @param objReader Shared object reader.
+   * @param revWalk Shared rev walk.
+   * @throws IOException if the update failed.
+   */
+  public BatchMetaDataUpdate openUpdate(
+      MetaDataUpdate update, ObjectInserter objInserter, ObjectReader objReader, RevWalk revWalk)
+      throws IOException {
     final Repository db = update.getRepository();
 
-    inserter = db.newObjectInserter();
-    reader = inserter.newReader();
-    final RevWalk rw = new RevWalk(reader);
+    inserter = objInserter == null ? db.newObjectInserter() : objInserter;
+    reader = objReader == null ? inserter.newReader() : objReader;
+    final RevWalk rw = revWalk == null ? new RevWalk(reader) : revWalk;
+
     final RevTree tree = revision != null ? rw.parseTree(revision) : null;
     newTree = readTree(tree);
     return new BatchMetaDataUpdate() {
@@ -376,7 +425,7 @@
         newTree = null;
 
         rw.close();
-        if (inserter != null) {
+        if (objInserter == null && inserter != null) {
           inserter.close();
           inserter = null;
         }
@@ -392,7 +441,9 @@
         BatchRefUpdate bru = update.getBatch();
         if (bru != null) {
           bru.addCommand(new ReceiveCommand(oldId.toObjectId(), newId.toObjectId(), refName));
-          inserter.flush();
+          if (objInserter == null) {
+            inserter.flush();
+          }
           revision = rw.parseCommit(newId);
           return revision;
         }
diff --git a/java/com/google/gerrit/server/index/AutoFlush.java b/java/com/google/gerrit/server/index/AutoFlush.java
new file mode 100644
index 0000000..3d13110
--- /dev/null
+++ b/java/com/google/gerrit/server/index/AutoFlush.java
@@ -0,0 +1,20 @@
+// Copyright (C) 2021 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.server.index;
+
+public enum AutoFlush {
+  ENABLED,
+  DISABLED
+}
diff --git a/java/com/google/gerrit/server/index/IndexModule.java b/java/com/google/gerrit/server/index/IndexModule.java
index e7b892d..612c637 100644
--- a/java/com/google/gerrit/server/index/IndexModule.java
+++ b/java/com/google/gerrit/server/index/IndexModule.java
@@ -224,13 +224,14 @@
       return interactiveExecutor;
     }
     int threads = this.threads;
-    if (threads < 0) {
-      return MoreExecutors.newDirectExecutorService();
-    } else if (threads == 0) {
+    if (threads == 0) {
       threads =
           config.getInt(
               "index", null, "threads", Runtime.getRuntime().availableProcessors() / 2 + 1);
     }
+    if (threads < 0) {
+      return MoreExecutors.newDirectExecutorService();
+    }
     return MoreExecutors.listeningDecorator(
         workQueue.createQueue(threads, "Index-Interactive", true));
   }
@@ -243,11 +244,13 @@
     if (batchExecutor != null) {
       return batchExecutor;
     }
-    int threads = config.getInt("index", null, "batchThreads", 0);
+    int threads = this.threads;
+    if (threads == 0) {
+      threads =
+          config.getInt("index", null, "batchThreads", Runtime.getRuntime().availableProcessors());
+    }
     if (threads < 0) {
       return MoreExecutors.newDirectExecutorService();
-    } else if (threads == 0) {
-      threads = Runtime.getRuntime().availableProcessors();
     }
     return MoreExecutors.listeningDecorator(workQueue.createQueue(threads, "Index-Batch", true));
   }
diff --git a/java/com/google/gerrit/server/mail/send/OutgoingEmail.java b/java/com/google/gerrit/server/mail/send/OutgoingEmail.java
index 3bb710b..97035f2 100644
--- a/java/com/google/gerrit/server/mail/send/OutgoingEmail.java
+++ b/java/com/google/gerrit/server/mail/send/OutgoingEmail.java
@@ -21,6 +21,7 @@
 import com.google.common.collect.Sets;
 import com.google.common.flogger.FluentLogger;
 import com.google.gerrit.exceptions.EmailException;
+import com.google.gerrit.common.Nullable;
 import com.google.gerrit.extensions.api.changes.RecipientType;
 import com.google.gerrit.extensions.client.GeneralPreferencesInfo;
 import com.google.gerrit.extensions.client.GeneralPreferencesInfo.EmailFormat;
@@ -333,7 +334,7 @@
   }
 
   /** Lookup a human readable name for an account, usually the "full name". */
-  protected String getNameFor(Account.Id accountId) {
+  protected String getNameFor(@Nullable Account.Id accountId) {
     if (accountId == null) {
       return args.gerritPersonIdent.getName();
     }
@@ -359,7 +360,14 @@
    * @param accountId user to fetch.
    * @return name/email of account, or Anonymous Coward if unset.
    */
-  protected String getNameEmailFor(Account.Id accountId) {
+  protected String getNameEmailFor(@Nullable Account.Id accountId) {
+    if (accountId == null) {
+      return args.gerritPersonIdent.getName()
+          + " <"
+          + args.gerritPersonIdent.getEmailAddress()
+          + ">";
+    }
+
     Optional<Account> account = args.accountCache.get(accountId).map(AccountState::getAccount);
     if (account.isPresent()) {
       String name = account.get().getFullName();
@@ -380,9 +388,13 @@
    * username. If no username is set, this function returns null.
    *
    * @param accountId user to fetch.
-   * @return name/email of account, username, or null if unset.
+   * @return name/email of account, username, or null if unset or the accountId is null.
    */
-  protected String getUserNameEmailFor(Account.Id accountId) {
+  protected String getUserNameEmailFor(@Nullable Account.Id accountId) {
+    if (accountId == null) {
+      return null;
+    }
+
     Optional<AccountState> accountState = args.accountCache.get(accountId);
     if (!accountState.isPresent()) {
       return null;
diff --git a/java/com/google/gerrit/testing/InMemoryModule.java b/java/com/google/gerrit/testing/InMemoryModule.java
index 83b9e2b..9d07780 100644
--- a/java/com/google/gerrit/testing/InMemoryModule.java
+++ b/java/com/google/gerrit/testing/InMemoryModule.java
@@ -61,6 +61,7 @@
 import com.google.gerrit.server.git.PerThreadRequestScope;
 import com.google.gerrit.server.git.SearchingChangeCacheImpl;
 import com.google.gerrit.server.git.WorkQueue;
+import com.google.gerrit.server.index.AutoFlush;
 import com.google.gerrit.server.index.IndexModule.IndexType;
 import com.google.gerrit.server.index.account.AccountSchemaDefinitions;
 import com.google.gerrit.server.index.account.AllAccountsIndexer;
@@ -304,8 +305,13 @@
       boolean slave = cfg.getBoolean("container", "slave", false);
       Class<?> clazz = Class.forName(moduleClassName);
       Method m =
-          clazz.getMethod("singleVersionWithExplicitVersions", Map.class, int.class, boolean.class);
-      return (Module) m.invoke(null, getSingleSchemaVersions(), 0, slave);
+          clazz.getMethod(
+              "singleVersionWithExplicitVersions",
+              Map.class,
+              int.class,
+              boolean.class,
+              AutoFlush.class);
+      return (Module) m.invoke(null, getSingleSchemaVersions(), 0, slave, AutoFlush.ENABLED);
     } catch (ClassNotFoundException
         | SecurityException
         | NoSuchMethodException