KAFKA-15046: Get rid of unnecessary fsyncs inside UnifiedLog.lock to stabilize performance

clients/src/main/java/org/apache/kafka/common/utils/Utils.java

@@ -1020,6 +1020,17 @@ public static void flushDirIfExists(Path path) throws IOException {
         }
     }
 
+    /**
+     * Flushes dirty file with swallowing {@link NoSuchFileException}
+     */
+    public static void flushFileIfExists(Path path) throws IOException {
+        try (FileChannel fileChannel = FileChannel.open(path, StandardOpenOption.READ)) {
+            fileChannel.force(true);
+        } catch (NoSuchFileException e) {
+            log.warn("Failed to flush file {}", path, e);
+        }
+    }
+
     /**
      * Closes all the provided closeables.
      * @throws IOException if any of the close methods throws an IOException.
@@ -1497,7 +1508,7 @@ public static <S> Iterator<S> covariantCast(Iterator<? extends S> iterator) {
      * Checks if a string is null, empty or whitespace only.
      * @param str a string to be checked
      * @return true if the string is null, empty or whitespace only; otherwise, return false.
-     */    
+     */
     public static boolean isBlank(String str) {
         return str == null || str.trim().isEmpty();
     }

core/src/main/scala/kafka/log/UnifiedLog.scala

@@ -43,7 +43,7 @@ import org.apache.kafka.storage.internals.epoch.LeaderEpochFileCache
 import org.apache.kafka.storage.internals.log.{AbortedTxn, AppendOrigin, BatchMetadata, CompletedTxn, EpochEntry, FetchDataInfo, FetchIsolation, LastRecord, LeaderHwChange, LogAppendInfo, LogConfig, LogDirFailureChannel, LogFileUtils, LogOffsetMetadata, LogOffsetSnapshot, LogOffsetsListener, LogStartOffsetIncrementReason, LogValidator, ProducerAppendInfo, ProducerStateManager, ProducerStateManagerConfig, RollParams}
 
 import java.io.{File, IOException}
-import java.nio.file.Files
+import java.nio.file.{Files, Path}
 import java.util
 import java.util.concurrent.{ConcurrentHashMap, ConcurrentMap}
 import java.util.{Collections, Optional, OptionalInt, OptionalLong}
@@ -1617,10 +1617,16 @@ class UnifiedLog(@volatile var logStartOffset: Long,
     // may actually be ahead of the current producer state end offset (which corresponds to the log end offset),
     // we manually override the state offset here prior to taking the snapshot.
     producerStateManager.updateMapEndOffset(newSegment.baseOffset)
-    producerStateManager.takeSnapshot()
+    // We avoid potentially-costly fsync call, since we acquire UnifiedLog#lock here
+    // which could block subsequent produces in the meantime.
+    // flush is done in the scheduler thread along with segment flushing below
+    val maybeSnapshot = producerStateManager.takeSnapshot(false)
     updateHighWatermarkWithLogEndOffset()
     // Schedule an asynchronous flush of the old segment
-    scheduler.scheduleOnce("flush-log", () => flushUptoOffsetExclusive(newSegment.baseOffset))
+    scheduler.scheduleOnce("flush-log", () => {
+      maybeSnapshot.ifPresent(f => flushProducerStateSnapshot(f.toPath))
+      flushUptoOffsetExclusive(newSegment.baseOffset)
+    })
     newSegment
   }
 
@@ -1703,6 +1709,12 @@ class UnifiedLog(@volatile var logStartOffset: Long,
     producerStateManager.mapEndOffset
   }
 
+  private[log] def flushProducerStateSnapshot(snapshot: Path): Unit = {
+    maybeHandleIOException(s"Error while deleting producer state snapshot $snapshot for $topicPartition in dir ${dir.getParent}") {
+      Utils.flushFileIfExists(snapshot)
+    }
+  }
+
   /**
    * Truncate this log so that it ends with the greatest offset < targetOffset.
    *

core/src/main/scala/kafka/server/checkpoints/OffsetCheckpointFile.scala

@@ -68,7 +68,7 @@ class OffsetCheckpointFile(val file: File, logDirFailureChannel: LogDirFailureCh
   def write(offsets: Map[TopicPartition, Long]): Unit = {
     val list: java.util.List[(TopicPartition, Long)] = new java.util.ArrayList[(TopicPartition, Long)](offsets.size)
     offsets.foreach(x => list.add(x))
-    checkpoint.write(list)
+    checkpoint.write(list, true)
   }
 
   def read(): Map[TopicPartition, Long] = {

core/src/test/java/kafka/log/remote/RemoteLogManagerTest.java

@@ -162,7 +162,7 @@ public class RemoteLogManagerTest {
         List<EpochEntry> epochs = Collections.emptyList();
 
         @Override
-        public void write(Collection<EpochEntry> epochs) {
+        public void write(Collection<EpochEntry> epochs, boolean ignored) {
             this.epochs = new ArrayList<>(epochs);
         }
 

core/src/test/scala/unit/kafka/log/LogSegmentTest.scala

@@ -407,7 +407,7 @@ class LogSegmentTest {
     val checkpoint: LeaderEpochCheckpoint = new LeaderEpochCheckpoint {
       private var epochs = Seq.empty[EpochEntry]
 
-      override def write(epochs: util.Collection[EpochEntry]): Unit = {
+      override def write(epochs: util.Collection[EpochEntry], ignored: Boolean): Unit = {
         this.epochs = epochs.asScala.toSeq
       }
 

core/src/test/scala/unit/kafka/log/UnifiedLogTest.scala

@@ -40,8 +40,8 @@ import org.apache.kafka.storage.internals.log.{AbortedTxn, AppendOrigin, EpochEn
 import org.junit.jupiter.api.Assertions._
 import org.junit.jupiter.api.{AfterEach, BeforeEach, Test}
 import org.mockito.ArgumentMatchers
-import org.mockito.ArgumentMatchers.anyLong
-import org.mockito.Mockito.{mock, when}
+import org.mockito.ArgumentMatchers.{any, anyLong}
+import org.mockito.Mockito.{doThrow, mock, spy, when}
 
 import java.io._
 import java.nio.ByteBuffer
@@ -3581,7 +3581,7 @@ class UnifiedLogTest {
         val records = TestUtils.singletonRecords(value = s"test$i".getBytes)
         log.appendAsLeader(records, leaderEpoch = 0)
       }
-      
+
       log.updateHighWatermark(90L)
       log.maybeIncrementLogStartOffset(20L, LogStartOffsetIncrementReason.SegmentDeletion)
       assertEquals(20, log.logStartOffset)
@@ -3796,6 +3796,24 @@ class UnifiedLogTest {
     log.appendAsLeader(transactionalRecords, leaderEpoch = 0, verificationGuard = verificationGuard)
   }
 
+  @Test
+  def testRecoveryPointNotIncrementedOnProducerStateSnapshotFlushFailure(): Unit = {
+    val logConfig = LogTestUtils.createLogConfig()
+    val log = spy(createLog(logDir, logConfig))
+
+    doThrow(new KafkaStorageException("Injected exception")).when(log).flushProducerStateSnapshot(any())
+
+    log.appendAsLeader(TestUtils.singletonRecords("a".getBytes), leaderEpoch = 0)
+    try {
+      log.roll(Some(1L))
+    } catch {
+      case _: KafkaStorageException => // ignore
+    }
+
+    // check that the recovery point isn't incremented
+    assertEquals(0L, log.recoveryPoint)
+  }
+
   private def appendTransactionalToBuffer(buffer: ByteBuffer,
                                           producerId: Long,
                                           producerEpoch: Short,

core/src/test/scala/unit/kafka/server/checkpoints/OffsetCheckpointFileWithFailureHandlerTest.scala

@@ -97,7 +97,7 @@ class OffsetCheckpointFileWithFailureHandlerTest extends Logging {
     val logDirFailureChannel = new LogDirFailureChannel(10)
     val checkpointFile = new CheckpointFileWithFailureHandler(file, OffsetCheckpointFile.CurrentVersion + 1,
       OffsetCheckpointFile.Formatter, logDirFailureChannel, file.getParent)
-    checkpointFile.write(Collections.singletonList(new TopicPartition("foo", 5) -> 10L))
+    checkpointFile.write(Collections.singletonList(new TopicPartition("foo", 5) -> 10L), true)
     assertThrows(classOf[KafkaStorageException], () => new OffsetCheckpointFile(checkpointFile.file, logDirFailureChannel).read())
   }
 

core/src/test/scala/unit/kafka/server/epoch/LeaderEpochFileCacheTest.scala

@@ -38,7 +38,7 @@ class LeaderEpochFileCacheTest {
   val tp = new TopicPartition("TestTopic", 5)
   private val checkpoint: LeaderEpochCheckpoint = new LeaderEpochCheckpoint {
     private var epochs: Seq[EpochEntry] = Seq()
-    override def write(epochs: java.util.Collection[EpochEntry]): Unit = this.epochs = epochs.asScala.toSeq
+    override def write(epochs: java.util.Collection[EpochEntry], ignored: Boolean): Unit = this.epochs = epochs.asScala.toSeq
     override def read(): java.util.List[EpochEntry] = this.epochs.asJava
   }
 

server-common/src/main/java/org/apache/kafka/server/common/CheckpointFile.java

@@ -72,18 +72,20 @@ public CheckpointFile(File file,
         tempPath = Paths.get(absolutePath + ".tmp");
     }
 
-    public void write(Collection<T> entries) throws IOException {
+    public void write(Collection<T> entries, boolean sync) throws IOException {
         synchronized (lock) {
             // write to temp file and then swap with the existing file
             try (FileOutputStream fileOutputStream = new FileOutputStream(tempPath.toFile());
                  BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(fileOutputStream, StandardCharsets.UTF_8))) {
                 CheckpointWriteBuffer<T> checkpointWriteBuffer = new CheckpointWriteBuffer<>(writer, version, formatter);
                 checkpointWriteBuffer.write(entries);
                 writer.flush();
-                fileOutputStream.getFD().sync();
+                if (sync) {
+                    fileOutputStream.getFD().sync();
+                }
             }
 
-            Utils.atomicMoveWithFallback(tempPath, absolutePath);
+            Utils.atomicMoveWithFallback(tempPath, absolutePath, sync);
         }
     }
 

storage/src/main/java/org/apache/kafka/server/log/remote/metadata/storage/CommittedOffsetsFile.java

@@ -68,7 +68,7 @@ public Optional<Map.Entry<Integer, Long>> fromString(String line) {
     }
 
     public synchronized void writeEntries(Map<Integer, Long> committedOffsets) throws IOException {
-        checkpointFile.write(committedOffsets.entrySet());
+        checkpointFile.write(committedOffsets.entrySet(), true);
     }
 
     public synchronized Map<Integer, Long> readEntries() throws IOException {
@@ -83,4 +83,4 @@ public synchronized Map<Integer, Long> readEntries() throws IOException {
 
         return partitionToOffsets;
     }
-}
+}

storage/src/main/java/org/apache/kafka/storage/internals/checkpoint/CheckpointFileWithFailureHandler.java

@@ -41,9 +41,9 @@ public CheckpointFileWithFailureHandler(File file, int version, CheckpointFile.E
         checkpointFile = new CheckpointFile<>(file, version, formatter);
     }
 
-    public void write(Collection<T> entries) {
+    public void write(Collection<T> entries, boolean sync) {
         try {
-            checkpointFile.write(entries);
+            checkpointFile.write(entries, sync);
         } catch (IOException e) {
             String msg = "Error while writing to checkpoint file " + file.getAbsolutePath();
             logDirFailureChannel.maybeAddOfflineLogDir(logDir, msg, e);

storage/src/main/java/org/apache/kafka/storage/internals/checkpoint/InMemoryLeaderEpochCheckpoint.java

@@ -42,7 +42,7 @@
 public class InMemoryLeaderEpochCheckpoint implements LeaderEpochCheckpoint {
     private List<EpochEntry> epochs = Collections.emptyList();
 
-    public void write(Collection<EpochEntry> epochs) {
+    public void write(Collection<EpochEntry> epochs, boolean ignored) {
         this.epochs = new ArrayList<>(epochs);
     }
 
@@ -60,4 +60,4 @@ public ByteBuffer readAsByteBuffer() throws IOException {
 
         return ByteBuffer.wrap(stream.toByteArray());
     }
-}
+}

storage/src/main/java/org/apache/kafka/storage/internals/checkpoint/LeaderEpochCheckpoint.java

@@ -22,8 +22,13 @@
 import java.util.List;
 
 public interface LeaderEpochCheckpoint {
+    // in file-backed checkpoint implementation, the content should be
+    // synced to the device if `sync` is true
+    void write(Collection<EpochEntry> epochs, boolean sync);
 
-    void write(Collection<EpochEntry> epochs);
+    default void write(Collection<EpochEntry> epochs) {
+        write(epochs, true);
+    }
 
     List<EpochEntry> read();
 }

storage/src/main/java/org/apache/kafka/storage/internals/checkpoint/LeaderEpochCheckpointFile.java

@@ -53,7 +53,11 @@ public LeaderEpochCheckpointFile(File file, LogDirFailureChannel logDirFailureCh
     }
 
     public void write(Collection<EpochEntry> epochs) {
-        checkpoint.write(epochs);
+        write(epochs, true);
+    }
+
+    public void write(Collection<EpochEntry> epochs, boolean sync) {
+        checkpoint.write(epochs, sync);
     }
 
     public List<EpochEntry> read() {
@@ -75,4 +79,4 @@ public Optional<EpochEntry> fromString(String line) {
             return (strings.length == 2) ? Optional.of(new EpochEntry(Integer.parseInt(strings[0]), Long.parseLong(strings[1]))) : Optional.empty();
         }
     }
-}
+}

storage/src/main/java/org/apache/kafka/storage/internals/epoch/LeaderEpochFileCache.java

@@ -72,7 +72,7 @@ public void assign(int epoch, long startOffset) {
         EpochEntry entry = new EpochEntry(epoch, startOffset);
         if (assign(entry)) {
             log.debug("Appended new epoch entry {}. Cache now contains {} entries.", entry, epochs.size());
-            flush();
+            writeToFile(true);
         }
     }
 
@@ -82,7 +82,7 @@ public void assign(List<EpochEntry> entries) {
                 log.debug("Appended new epoch entry {}. Cache now contains {} entries.", entry, epochs.size());
             }
         });
-        if (!entries.isEmpty()) flush();
+        if (!entries.isEmpty()) writeToFile(true);
     }
 
     private boolean isUpdateNeeded(EpochEntry entry) {
@@ -151,11 +151,6 @@ private List<EpochEntry> removeWhileMatching(Iterator<Map.Entry<Integer, EpochEn
         return removedEpochs;
     }
 
-    public LeaderEpochFileCache cloneWithLeaderEpochCheckpoint(LeaderEpochCheckpoint leaderEpochCheckpoint) {
-        flushTo(leaderEpochCheckpoint);
-        return new LeaderEpochFileCache(this.topicPartition, leaderEpochCheckpoint);
-    }
-
     public boolean nonEmpty() {
         lock.readLock().lock();
         try {
@@ -308,7 +303,14 @@ public void truncateFromEnd(long endOffset) {
             if (endOffset >= 0 && epochEntry.isPresent() && epochEntry.get().startOffset >= endOffset) {
                 List<EpochEntry> removedEntries = removeFromEnd(x -> x.startOffset >= endOffset);
 
-                flush();
+                // We intentionally don't force flushing change to the device here because:
+                // - To avoid fsync latency
+                //   * fsync latency could be huge on a disk glitch, which is not rare in spinning drives
+                //   * This method is called by ReplicaFetcher threads, which could block replica fetching
+                //     then causing ISR shrink or high produce response time degradation in remote scope on high fsync latency.
+                // - Even when stale epochs remained in LeaderEpoch file due to the unclean shutdown, it will be handled by
+                //   another truncateFromEnd call on log loading procedure so it won't be a problem
+                writeToFile(false);
 
                 log.debug("Cleared entries {} from epoch cache after truncating to end offset {}, leaving {} entries in the cache.", removedEntries, endOffset, epochs.size());
             }
@@ -335,7 +337,14 @@ public void truncateFromStart(long startOffset) {
                 EpochEntry updatedFirstEntry = new EpochEntry(firstBeforeStartOffset.epoch, startOffset);
                 epochs.put(updatedFirstEntry.epoch, updatedFirstEntry);
 
-                flush();
+                // We intentionally don't force flushing change to the device here because:
+                // - To avoid fsync latency
+                //   * fsync latency could be huge on a disk glitch, which is not rare in spinning drives
+                //   * This method is called as part of deleteRecords with holding UnifiedLog#lock.
+                //      - Meanwhile all produces against the partition will be blocked, which causes req-handlers to exhaust
+                // - Even when stale epochs remained in LeaderEpoch file due to the unclean shutdown, it will be recovered by
+                //   another truncateFromStart call on log loading procedure so it won't be a problem
+                writeToFile(false);
 
                 log.debug("Cleared entries {} and rewrote first entry {} after truncating to start offset {}, leaving {} in the cache.", removedEntries, updatedFirstEntry, startOffset, epochs.size());
             }
@@ -384,7 +393,7 @@ public void clearAndFlush() {
         lock.writeLock().lock();
         try {
             epochs.clear();
-            flush();
+            writeToFile(true);
         } finally {
             lock.writeLock().unlock();
         }
@@ -421,16 +430,12 @@ public NavigableMap<Integer, Long> epochWithOffsets() {
         }
     }
 
-    private void flushTo(LeaderEpochCheckpoint leaderEpochCheckpoint) {
+    private void writeToFile(boolean sync) {
         lock.readLock().lock();
         try {
-            leaderEpochCheckpoint.write(epochs.values());
+            checkpoint.write(epochs.values(), sync);
         } finally {
             lock.readLock().unlock();
         }
     }
-
-    private void flush() {
-        flushTo(this.checkpoint);
-    }
 }