Include bytes for live docs in ShardFieldStats

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -32,7 +32,9 @@
 import org.apache.lucene.search.ReferenceManager;
 import org.apache.lucene.search.similarities.Similarity;
 import org.apache.lucene.store.AlreadyClosedException;
+import org.apache.lucene.util.Bits;
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.RamUsageEstimator;
 import org.apache.lucene.util.SetOnce;
 import org.elasticsearch.ExceptionsHelper;
 import org.elasticsearch.action.ActionListener;
@@ -280,6 +282,7 @@ protected static ShardFieldStats shardFieldStats(List<LeafReaderContext> leaves)
         int totalFields = 0;
         long usages = 0;
         long totalPostingBytes = 0;
+        long liveDocsBytes = 0;
         for (LeafReaderContext leaf : leaves) {
             numSegments++;
             var fieldInfos = leaf.reader().getFieldInfos();
@@ -291,19 +294,44 @@ protected static ShardFieldStats shardFieldStats(List<LeafReaderContext> leaves)
             } else {
                 usages = -1;
             }
-            if (TrackingPostingsInMemoryBytesCodec.TRACK_POSTINGS_IN_MEMORY_BYTES.isEnabled()) {
+            boolean trackPostingsMemoryEnabled = TrackingPostingsInMemoryBytesCodec.TRACK_POSTINGS_IN_MEMORY_BYTES.isEnabled();
+            boolean trackLiveDocsMemoryEnabled = ShardFieldStats.TRACK_LIVE_DOCS_IN_MEMORY_BYTES.isEnabled();
+            if (trackLiveDocsMemoryEnabled || trackPostingsMemoryEnabled) {
                 SegmentReader segmentReader = Lucene.tryUnwrapSegmentReader(leaf.reader());
                 if (segmentReader != null) {
-                    String postingBytes = segmentReader.getSegmentInfo().info.getAttribute(
-                        TrackingPostingsInMemoryBytesCodec.IN_MEMORY_POSTINGS_BYTES_KEY
-                    );
-                    if (postingBytes != null) {
-                        totalPostingBytes += Long.parseLong(postingBytes);
+                    if (trackPostingsMemoryEnabled) {
+                        String postingBytes = segmentReader.getSegmentInfo().info.getAttribute(
+                            TrackingPostingsInMemoryBytesCodec.IN_MEMORY_POSTINGS_BYTES_KEY
+                        );
+                        if (postingBytes != null) {
+                            totalPostingBytes += Long.parseLong(postingBytes);
+                        }
+                    }
+                    if (trackLiveDocsMemoryEnabled) {
+                        var liveDocs = segmentReader.getLiveDocs();
+                        if (liveDocs != null) {
+                            assert validateLiveDocsClass(liveDocs);
+                            // Would prefer to use FixedBitSet#ramBytesUsed() however FixedBits / Bits interface don't expose that.
+                            // This almost does what FixedBitSet#ramBytesUsed() does, liveDocs.length() returns the length of the bits long
+                            // array
+                            liveDocsBytes += RamUsageEstimator.alignObjectSize(
+                                (long) RamUsageEstimator.NUM_BYTES_ARRAY_HEADER + (liveDocs.length() / 8L)
+                            );
+                        }
                     }
                 }
             }
         }
-        return new ShardFieldStats(numSegments, totalFields, usages, totalPostingBytes);
+        return new ShardFieldStats(numSegments, totalFields, usages, totalPostingBytes, liveDocsBytes);
+    }
+
+    private static boolean validateLiveDocsClass(Bits liveDocs) {
+        // These classes are package protected in Lucene and therefor we compare fully qualified classnames as strings here:
+        String fullClassName = liveDocs.getClass().getName();
+        assert fullClassName.equals("org.apache.lucene.util.FixedBits")
+            || fullClassName.equals("org.apache.lucene.tests.codecs.asserting.AssertingLiveDocsFormat$AssertingBits")
+            : "unexpected class [" + fullClassName + "]";
+        return true;
     }
 
     /**

server/src/main/java/org/elasticsearch/index/shard/ShardFieldStats.java

@@ -9,16 +9,21 @@
 
 package org.elasticsearch.index.shard;
 
+import org.elasticsearch.common.util.FeatureFlag;
+
 /**
  * A per shard stats including the number of segments and total fields across those segments.
  * These stats should be recomputed whenever the shard is refreshed.
  *
- * @param numSegments the number of segments
- * @param totalFields the total number of fields across the segments
- * @param fieldUsages the number of usages for segment-level fields (e.g., doc_values, postings, norms, points)
- *                    -1 if unavailable
+ * @param numSegments           the number of segments
+ * @param totalFields           the total number of fields across the segments
+ * @param fieldUsages           the number of usages for segment-level fields (e.g., doc_values, postings, norms, points)
+ *                              -1 if unavailable
  * @param postingsInMemoryBytes the total bytes in memory used for postings across all fields
+ * @param liveDocsBytes         the total bytes in memory used for live docs
  */
-public record ShardFieldStats(int numSegments, int totalFields, long fieldUsages, long postingsInMemoryBytes) {
+public record ShardFieldStats(int numSegments, int totalFields, long fieldUsages, long postingsInMemoryBytes, long liveDocsBytes) {
+
+    public static final FeatureFlag TRACK_LIVE_DOCS_IN_MEMORY_BYTES = new FeatureFlag("track_live_docs_in_memory_bytes");
 
 }

server/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -1980,6 +1980,54 @@ public void testShardFieldStats() throws IOException {
         closeShards(shard);
     }
 
+    public void testShardFieldStatsWithDeletes() throws IOException {
+        Settings settings = Settings.builder().put(IndexSettings.INDEX_REFRESH_INTERVAL_SETTING.getKey(), TimeValue.MINUS_ONE).build();
+        IndexShard shard = newShard(true, settings);
+        assertNull(shard.getShardFieldStats());
+        recoverShardFromStore(shard);
+        boolean liveDocsTrackingEnabled = ShardFieldStats.TRACK_LIVE_DOCS_IN_MEMORY_BYTES.isEnabled();
+
+        // index some documents
+        int numDocs = 10;
+        for (int i = 0; i < numDocs; i++) {
+            indexDoc(shard, "_doc", "first_" + i, """
+                {
+                    "f1": "foo",
+                    "f2": "bar"
+                }
+                """);
+        }
+        shard.refresh("test");
+        var stats = shard.getShardFieldStats();
+        assertThat(stats.numSegments(), equalTo(1));
+        assertThat(stats.liveDocsBytes(), equalTo(0L));
+
+        // delete a doc
+        deleteDoc(shard, "first_0");
+
+        // Refresh and fetch new stats:
+        shard.refresh("test");
+        stats = shard.getShardFieldStats();
+        // More segments because delete operation is stored in the new segment for replication purposes.
+        assertThat(stats.numSegments(), equalTo(2));
+        // Delete op is stored in new segment, but marked as deleted. All segements have live docs:
+        assertThat(stats.liveDocsBytes(), equalTo(liveDocsTrackingEnabled ? 40L : 0L));
+
+        // delete another doc:
+        deleteDoc(shard, "first_1");
+        shard.getMinRetainedSeqNo();
+
+        // Refresh and fetch new stats:
+        shard.refresh("test");
+        stats = shard.getShardFieldStats();
+        // More segments because delete operation is stored in the new segment for replication purposes.
+        assertThat(stats.numSegments(), equalTo(3));
+        // Delete op is stored in new segment, but marked as deleted. All segements have live docs:
+        assertThat(stats.liveDocsBytes(), equalTo(liveDocsTrackingEnabled ? 56L : 0L));
+
+        closeShards(shard);
+    }
+
     public void testIndexingOperationsListeners() throws IOException {
         IndexShard shard = newStartedShard(true);
         indexDoc(shard, "_doc", "0", "{\"foo\" : \"bar\"}");