Add lower bound for translog flush threshold (#28382)

If the translog flush threshold is too small (eg. smaller than the
translog header), we may repeatedly flush even there is no uncommitted
operation because the shouldFlush condition can still be true after
flushing. This is currently avoided by adding an extra guard against the
uncommitted operations. However, this extra guard makes the shouldFlush
complicated. This commit replaces that extra guard by a lower bound for
translog flush threshold. We keep the lower bound small for convenience
in testing.

Relates #28350
Relates #23606

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

@@ -186,8 +186,15 @@ public final class IndexSettings {
         Setting.timeSetting("index.refresh_interval", DEFAULT_REFRESH_INTERVAL, new TimeValue(-1, TimeUnit.MILLISECONDS),
             Property.Dynamic, Property.IndexScope);
     public static final Setting<ByteSizeValue> INDEX_TRANSLOG_FLUSH_THRESHOLD_SIZE_SETTING =
-        Setting.byteSizeSetting("index.translog.flush_threshold_size", new ByteSizeValue(512, ByteSizeUnit.MB), Property.Dynamic,
-            Property.IndexScope);
+        Setting.byteSizeSetting("index.translog.flush_threshold_size", new ByteSizeValue(512, ByteSizeUnit.MB),
+            /*
+             * An empty translog occupies 43 bytes on disk. If the flush threshold is below this, the flush thread
+             * can get stuck in an infinite loop as the shouldPeriodicallyFlush can still be true after flushing.
+             * However, small thresholds are useful for testing so we do not add a large lower bound here.
+             */
+            new ByteSizeValue(Translog.DEFAULT_HEADER_SIZE_IN_BYTES + 1, ByteSizeUnit.BYTES),
+            new ByteSizeValue(Long.MAX_VALUE, ByteSizeUnit.BYTES),
+            Property.Dynamic, Property.IndexScope);
 
     /**
      * Controls how long translog files that are no longer needed for persistence reasons
@@ -222,9 +229,9 @@ public final class IndexSettings {
                      * generation threshold. However, small thresholds are useful for testing so we
                      * do not add a large lower bound here.
                      */
-                    new ByteSizeValue(64, ByteSizeUnit.BYTES),
+                    new ByteSizeValue(Translog.DEFAULT_HEADER_SIZE_IN_BYTES + 1, ByteSizeUnit.BYTES),
                     new ByteSizeValue(Long.MAX_VALUE, ByteSizeUnit.BYTES),
-                    new Property[]{Property.Dynamic, Property.IndexScope});
+                    Property.Dynamic, Property.IndexScope);
 
     /**
      * Index setting to enable / disable deletes garbage collection.

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

@@ -1526,21 +1526,16 @@ public boolean shouldPeriodicallyFlush() {
         if (uncommittedSizeOfCurrentCommit < flushThreshold) {
             return false;
         }
+        assert translog.uncommittedOperations() > 0 : "translog required to flush periodically but not contain any uncommitted operation; "
+            + "uncommitted translog size [" + uncommittedSizeOfCurrentCommit + "], flush threshold [" + flushThreshold + "]";
         /*
          * We should only flush ony if the shouldFlush condition can become false after flushing.
          * This condition will change if the `uncommittedSize` of the new commit is smaller than
          * the `uncommittedSize` of the current commit. This method is to maintain translog only,
          * thus the IndexWriter#hasUncommittedChanges condition is not considered.
          */
         final long uncommittedSizeOfNewCommit = translog.sizeOfGensAboveSeqNoInBytes(localCheckpointTracker.getCheckpoint() + 1);
-        /*
-         * If flushThreshold is too small, we may repeatedly flush even there is no uncommitted operation
-         * as #sizeOfGensAboveSeqNoInByte and #uncommittedSizeInBytes can return different values.
-         * An empty translog file has non-zero `uncommittedSize` (the translog header), and method #sizeOfGensAboveSeqNoInBytes can
-         * return 0 now(no translog gen contains ops above local checkpoint) but method #uncommittedSizeInBytes will return an actual
-         * non-zero value after rolling a new translog generation. This can be avoided by checking the actual uncommitted operations.
-         */
-        return uncommittedSizeOfNewCommit < uncommittedSizeOfCurrentCommit && translog.uncommittedOperations() > 0;
+        return uncommittedSizeOfNewCommit < uncommittedSizeOfCurrentCommit;
     }
 
     @Override

server/src/main/java/org/elasticsearch/index/translog/Translog.java

@@ -108,6 +108,7 @@ public class Translog extends AbstractIndexShardComponent implements IndexShardC
     public static final String CHECKPOINT_FILE_NAME = "translog" + CHECKPOINT_SUFFIX;
 
     static final Pattern PARSE_STRICT_ID_PATTERN = Pattern.compile("^" + TRANSLOG_FILE_PREFIX + "(\\d+)(\\.tlog)$");
+    public static final int DEFAULT_HEADER_SIZE_IN_BYTES = TranslogWriter.getHeaderLength(UUIDs.randomBase64UUID());
 
     // the list of translog readers is guaranteed to be in order of translog generation
     private final List<TranslogReader> readers = new ArrayList<>();
@@ -451,7 +452,10 @@ public long sizeOfGensAboveSeqNoInBytes(long minSeqNo) {
      * @throws IOException if creating the translog failed
      */
     TranslogWriter createWriter(long fileGeneration) throws IOException {
-        return createWriter(fileGeneration, getMinFileGeneration(), globalCheckpointSupplier.getAsLong());
+        final TranslogWriter writer = createWriter(fileGeneration, getMinFileGeneration(), globalCheckpointSupplier.getAsLong());
+        assert writer.sizeInBytes() == DEFAULT_HEADER_SIZE_IN_BYTES : "Mismatch translog header size; " +
+            "empty translog size [" + writer.sizeInBytes() + ", header size [" + DEFAULT_HEADER_SIZE_IN_BYTES + "]";
+        return writer;
     }
 
     /**

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

@@ -367,7 +367,7 @@ public void testMaybeFlush() throws Exception {
         });
         assertEquals(0, translog.uncommittedOperations());
         translog.sync();
-        long size = translog.uncommittedSizeInBytes();
+        long size = Math.max(translog.uncommittedSizeInBytes(), Translog.DEFAULT_HEADER_SIZE_IN_BYTES + 1);
         logger.info("--> current translog size: [{}] num_ops [{}] generation [{}]", translog.uncommittedSizeInBytes(),
             translog.uncommittedOperations(), translog.getGeneration());
         client().admin().indices().prepareUpdateSettings("test").setSettings(Settings.builder().put(