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HBASE-19024 Provide a configurable option to hsync WAL edits to the disk for better durability (Harshal Jain)

This commit is contained in:
Andrew Purtell 2018-03-20 16:59:25 -07:00
parent e7cfdac7b5
commit 17ac2fe9c1
19 changed files with 277 additions and 50 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
hbase-client/src/main/java/org/apache/hadoop/hbase/client/Durability.java
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@ -50,7 +50,6 @@ public enum Durability {
SYNC_WAL,
/**
* Write the Mutation to the WAL synchronously and force the entries to disk.
* (Note: this is currently not supported and will behave identical to {@link #SYNC_WAL})
* See <a href="https://issues.apache.org/jira/browse/HADOOP-6313">HADOOP-6313</a>
*/
FSYNC_WAL

30
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HRegion.java
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@ -230,16 +230,19 @@ public class HRegion implements HeapSize, PropagatingConfigurationObserver, Regi
public static final String HBASE_MAX_CELL_SIZE_KEY = "hbase.server.keyvalue.maxsize";
public static final int DEFAULT_MAX_CELL_SIZE = 10485760;
public static final String HBASE_REGIONSERVER_MINIBATCH_SIZE =
"hbase.regionserver.minibatch.size";
public static final int DEFAULT_HBASE_REGIONSERVER_MINIBATCH_SIZE = 20000;
/**
* This is the global default value for durability. All tables/mutations not
* defining a durability or using USE_DEFAULT will default to this value.
*/
private static final Durability DEFAULT_DURABILITY = Durability.SYNC_WAL;
public static final String HBASE_REGIONSERVER_MINIBATCH_SIZE =
"hbase.regionserver.minibatch.size";
public static final int DEFAULT_HBASE_REGIONSERVER_MINIBATCH_SIZE = 20000;
public static final String WAL_HSYNC_CONF_KEY = "hbase.wal.hsync";
public static final boolean DEFAULT_WAL_HSYNC = false;
final AtomicBoolean closed = new AtomicBoolean(false);
/* Closing can take some time; use the closing flag if there is stuff we don't
@ -795,11 +798,19 @@ public class HRegion implements HeapSize, PropagatingConfigurationObserver, Regi
*/
this.rowProcessorTimeout = conf.getLong(
"hbase.hregion.row.processor.timeout", DEFAULT_ROW_PROCESSOR_TIMEOUT);
this.regionDurability = htd.getDurability() == Durability.USE_DEFAULT ?
DEFAULT_DURABILITY : htd.getDurability();
this.storeHotnessProtector = new StoreHotnessProtector(this, conf);
boolean forceSync = conf.getBoolean(WAL_HSYNC_CONF_KEY, DEFAULT_WAL_HSYNC);
/**
* This is the global default value for durability. All tables/mutations not defining a
* durability or using USE_DEFAULT will default to this value.
*/
Durability defaultDurability = forceSync ? Durability.FSYNC_WAL : Durability.SYNC_WAL;
this.regionDurability =
this.htableDescriptor.getDurability() == Durability.USE_DEFAULT ? defaultDurability :
this.htableDescriptor.getDurability();
if (rsServices != null) {
this.rsAccounting = this.rsServices.getRegionServerAccounting();
// don't initialize coprocessors if not running within a regionserver
@ -8361,10 +8372,11 @@ public class HRegion implements HeapSize, PropagatingConfigurationObserver, Regi
// nothing do to
break;
case SYNC_WAL:
this.wal.sync(txid, false);
break;
case FSYNC_WAL:
// sync the WAL edit (SYNC and FSYNC treated the same for now)
this.wal.sync(txid);
break;
this.wal.sync(txid, true);
break;
default:
throw new RuntimeException("Unknown durability " + durability);
}

37
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/FSHLog.java
View File

@ -41,6 +41,7 @@ import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hbase.HConstants;
import org.apache.hadoop.hbase.client.RegionInfo;
import org.apache.hadoop.hbase.regionserver.HRegion;
import org.apache.hadoop.hbase.trace.TraceUtil;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.hbase.util.ClassSize;
@ -129,6 +130,8 @@ public class FSHLog extends AbstractFSWAL<Writer> {
// Minimum tolerable replicas, if the actual value is lower than it, rollWriter will be triggered
private final int minTolerableReplication;
private final boolean useHsync;
// If live datanode count is lower than the default replicas value,
// RollWriter will be triggered in each sync(So the RollWriter will be
// triggered one by one in a short time). Using it as a workaround to slow
@ -212,6 +215,8 @@ public class FSHLog extends AbstractFSWAL<Writer> {
5);
this.closeErrorsTolerated = conf.getInt("hbase.regionserver.logroll.errors.tolerated", 2);
this.useHsync = conf.getBoolean(HRegion.WAL_HSYNC_CONF_KEY, HRegion.DEFAULT_WAL_HSYNC);
// rollWriter sets this.hdfs_out if it can.
rollWriter();
@ -255,7 +260,7 @@ public class FSHLog extends AbstractFSWAL<Writer> {
private void preemptiveSync(final ProtobufLogWriter nextWriter) {
long startTimeNanos = System.nanoTime();
try {
nextWriter.sync();
nextWriter.sync(useHsync);
postSync(System.nanoTime() - startTimeNanos, 0);
} catch (IOException e) {
// optimization failed, no need to abort here.
@ -332,7 +337,7 @@ public class FSHLog extends AbstractFSWAL<Writer> {
// sequence and zigzagLatch will be set together
assert sequence > 0L : "Failed to get sequence from ring buffer";
TraceUtil.addTimelineAnnotation("awaiting safepoint");
syncFuture = zigzagLatch.waitSafePoint(publishSyncOnRingBuffer(sequence));
syncFuture = zigzagLatch.waitSafePoint(publishSyncOnRingBuffer(sequence, false));
}
} catch (FailedSyncBeforeLogCloseException e) {
// If unflushed/unsynced entries on close, it is reason to abort.
@ -577,7 +582,7 @@ public class FSHLog extends AbstractFSWAL<Writer> {
Throwable lastException = null;
try {
TraceUtil.addTimelineAnnotation("syncing writer");
writer.sync();
writer.sync(useHsync);
TraceUtil.addTimelineAnnotation("writer synced");
currentSequence = updateHighestSyncedSequence(currentSequence);
} catch (IOException e) {
@ -684,15 +689,15 @@ public class FSHLog extends AbstractFSWAL<Writer> {
return this.disruptor.getRingBuffer().next();
}
private SyncFuture publishSyncOnRingBuffer() {
private SyncFuture publishSyncOnRingBuffer(boolean forceSync) {
long sequence = getSequenceOnRingBuffer();
return publishSyncOnRingBuffer(sequence);
return publishSyncOnRingBuffer(sequence, forceSync);
}
@VisibleForTesting
protected SyncFuture publishSyncOnRingBuffer(long sequence) {
protected SyncFuture publishSyncOnRingBuffer(long sequence, boolean forceSync) {
// here we use ring buffer sequence as transaction id
SyncFuture syncFuture = getSyncFuture(sequence);
SyncFuture syncFuture = getSyncFuture(sequence).setForceSync(forceSync);
try {
RingBufferTruck truck = this.disruptor.getRingBuffer().get(sequence);
truck.load(syncFuture);
@ -703,8 +708,8 @@ public class FSHLog extends AbstractFSWAL<Writer> {
}
// Sync all known transactions
private void publishSyncThenBlockOnCompletion(TraceScope scope) throws IOException {
SyncFuture syncFuture = publishSyncOnRingBuffer();
private void publishSyncThenBlockOnCompletion(TraceScope scope, boolean forceSync) throws IOException {
SyncFuture syncFuture = publishSyncOnRingBuffer(forceSync);
blockOnSync(syncFuture);
}
@ -731,19 +736,29 @@ public class FSHLog extends AbstractFSWAL<Writer> {
@Override
public void sync() throws IOException {
sync(useHsync);
}
@Override
public void sync(boolean forceSync) throws IOException {
try (TraceScope scope = TraceUtil.createTrace("FSHLog.sync")) {
publishSyncThenBlockOnCompletion(scope);
publishSyncThenBlockOnCompletion(scope, forceSync);
}
}
@Override
public void sync(long txid) throws IOException {
sync(txid, useHsync);
}
@Override
public void sync(long txid, boolean forceSync) throws IOException {
if (this.highestSyncedTxid.get() >= txid) {
// Already sync'd.
return;
}
try (TraceScope scope = TraceUtil.createTrace("FSHLog.sync")) {
publishSyncThenBlockOnCompletion(scope);
publishSyncThenBlockOnCompletion(scope, forceSync);
}
}

19
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/ProtobufLogWriter.java
View File

@ -72,11 +72,15 @@ public class ProtobufLogWriter extends AbstractProtobufLogWriter
}
@Override
public void sync() throws IOException {
public void sync(boolean forceSync) throws IOException {
FSDataOutputStream fsdos = this.output;
if (fsdos == null) return; // Presume closed
fsdos.flush();
fsdos.hflush();
if (forceSync) {
fsdos.hsync();
} else {
fsdos.hflush();
}
}
public FSDataOutputStream getStream() {
@ -89,10 +93,13 @@ public class ProtobufLogWriter extends AbstractProtobufLogWriter
short replication, long blockSize) throws IOException, StreamLacksCapabilityException {
this.output = fs.createNonRecursive(path, overwritable, bufferSize, replication, blockSize,
null);
// TODO Be sure to add a check for hsync if this branch includes HBASE-19024
if (fs.getConf().getBoolean(CommonFSUtils.UNSAFE_STREAM_CAPABILITY_ENFORCE, true) &&
!(CommonFSUtils.hasCapability(output, "hflush"))) {
throw new StreamLacksCapabilityException("hflush");
if (fs.getConf().getBoolean(CommonFSUtils.UNSAFE_STREAM_CAPABILITY_ENFORCE, true)) {
if (!CommonFSUtils.hasCapability(output, "hflush")) {
throw new StreamLacksCapabilityException("hflush");
}
if (!CommonFSUtils.hasCapability(output, "hsync")) {
throw new StreamLacksCapabilityException("hsync");
}
}
}

11
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/SyncFuture.java
View File

@ -67,6 +67,8 @@ class SyncFuture {
private Thread t;
private boolean forceSync;
/**
* Call this method to clear old usage and get it ready for new deploy.
* @param txid the new transaction id
@ -97,6 +99,15 @@ class SyncFuture {
return this.txid;
}
synchronized boolean isForceSync() {
return forceSync;
}
synchronized SyncFuture setForceSync(boolean forceSync) {
this.forceSync = forceSync;
return this;
}
/**
* @param txid the transaction id at which this future 'completed'.
* @param t Can be null. Set if we are 'completing' on error (and this 't' is the error).

17
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WAL.java
View File

@ -139,6 +139,23 @@ public interface WAL extends Closeable, WALFileLengthProvider {
*/
void sync(long txid) throws IOException;
/**
* @param forceSync Flag to force sync rather than flushing to the buffer. Example - Hadoop hflush
* vs hsync.
*/
default void sync(boolean forceSync) throws IOException {
sync();
}
/**
* @param txid Transaction id to sync to.
* @param forceSync Flag to force sync rather than flushing to the buffer. Example - Hadoop hflush
* vs hsync.
*/
default void sync(long txid, boolean forceSync) throws IOException {
sync(txid);
}
/**
* WAL keeps track of the sequence numbers that are as yet not flushed im memstores
* in order to be able to do accounting to figure which WALs can be let go. This method tells WAL

2
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WALProvider.java
View File

@ -79,7 +79,7 @@ public interface WALProvider {
// Writers are used internally. Users outside of the WAL should be relying on the
// interface provided by WAL.
interface Writer extends WriterBase {
void sync() throws IOException;
void sync(boolean forceSync) throws IOException;
void append(WAL.Entry entry) throws IOException;
}

4
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestFailedAppendAndSync.java
View File

@ -138,11 +138,11 @@ public class TestFailedAppendAndSync {
}
@Override
public void sync() throws IOException {
public void sync(boolean forceSync) throws IOException {
if (throwSyncException) {
throw new IOException("FAKE! Failed to replace a bad datanode...");
}
w.sync();
w.sync(forceSync);
}
@Override

4
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestHRegion.java
View File

@ -1132,8 +1132,8 @@ public class TestHRegion {
}
@Override
public void sync() throws IOException {
w.sync();
public void sync(boolean forceSync) throws IOException {
w.sync(forceSync);
}
@Override

12
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/TestWALLockup.java
View File

@ -172,11 +172,11 @@ public class TestWALLockup {
}
@Override
public void sync() throws IOException {
public void sync(boolean forceSync) throws IOException {
if (throwException) {
throw new IOException("FAKE! Failed to replace a bad datanode...SYNC");
}
w.sync();
w.sync(forceSync);
}
@Override
@ -252,7 +252,7 @@ public class TestWALLockup {
dodgyWAL.append(region.getRegionInfo(), key, edit, true);
boolean exception = false;
try {
dodgyWAL.sync();
dodgyWAL.sync(false);
} catch (Exception e) {
exception = true;
}
@ -345,7 +345,7 @@ public class TestWALLockup {
protected void publishSyncOnRingBufferAndBlock(long sequence) {
try {
super.blockOnSync(super.publishSyncOnRingBuffer(sequence));
super.blockOnSync(super.publishSyncOnRingBuffer(sequence, false));
Assert.fail("Expect an IOException here.");
} catch (IOException ignore) {
// Here, we will get an IOException.
@ -362,7 +362,7 @@ public class TestWALLockup {
}
@Override
public void sync() throws IOException {
public void sync(boolean forceSync) throws IOException {
throw new IOException("FAKE! Failed to replace a bad datanode...SYNC");
}
@ -425,7 +425,7 @@ public class TestWALLockup {
try {
LOG.info("Call sync for testing whether RingBufferEventHandler is hanging.");
dodgyWAL.sync(); // Should not get a hang here, otherwise we will see timeout in this test.
dodgyWAL.sync(false); // Should not get a hang here, otherwise we will see timeout in this test.
Assert.fail("Expect an IOException here.");
} catch (IOException ignore) {
}

2
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/wal/AbstractTestWALReplay.java
View File

@ -1230,7 +1230,7 @@ public abstract class AbstractTestWALReplay {
for (FSWALEntry entry : entries) {
writer.append(entry);
}
writer.sync();
writer.sync(false);
writer.close();
}

4
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/wal/TestLogRollingNoCluster.java
View File

@ -67,9 +67,9 @@ public class TestLogRollingNoCluster {
/** ProtobufLogWriter that simulates higher latencies in sync() call */
public static class HighLatencySyncWriter extends ProtobufLogWriter {
@Override
public void sync() throws IOException {
public void sync(boolean forceSync) throws IOException {
Threads.sleep(ThreadLocalRandom.current().nextInt(10));
super.sync();
super.sync(forceSync);
Threads.sleep(ThreadLocalRandom.current().nextInt(10));
}
}

2
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/wal/TestProtobufLog.java
View File

@ -48,6 +48,6 @@ public class TestProtobufLog extends AbstractTestProtobufLog<WALProvider.Writer>
@Override
protected void sync(Writer writer) throws IOException {
writer.sync();
writer.sync(false);
}
}

161
hbase-server/src/test/java/org/apache/hadoop/hbase/regionserver/wal/TestWALDurability.java Normal file
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@ -0,0 +1,161 @@
/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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 org.apache.hadoop.hbase.regionserver.wal;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertNull;
import java.io.IOException;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.fs.FileSystem;
import org.apache.hadoop.fs.Path;
import org.apache.hadoop.hbase.HBaseClassTestRule;
import org.apache.hadoop.hbase.HBaseTestingUtility;
import org.apache.hadoop.hbase.TableName;
import org.apache.hadoop.hbase.client.Durability;
import org.apache.hadoop.hbase.client.Put;
import org.apache.hadoop.hbase.regionserver.ChunkCreator;
import org.apache.hadoop.hbase.regionserver.HRegion;
import org.apache.hadoop.hbase.regionserver.MemStoreLABImpl;
import org.apache.hadoop.hbase.testclassification.MediumTests;
import org.apache.hadoop.hbase.util.Bytes;
import org.apache.hadoop.hbase.wal.WAL;
import org.junit.Before;
import org.junit.ClassRule;
import org.junit.Rule;
import org.junit.Test;
import org.junit.experimental.categories.Category;
import org.junit.rules.TestName;
/**
* Tests for WAL write durability - hflush vs hsync
*/
@Category({ MediumTests.class })
public class TestWALDurability {
@ClassRule
public static final HBaseClassTestRule CLASS_RULE =
HBaseClassTestRule.forClass(TestWALDurability.class);
private static final String COLUMN_FAMILY = "MyCF";
private static final byte[] COLUMN_FAMILY_BYTES = Bytes.toBytes(COLUMN_FAMILY);
private final static HBaseTestingUtility TEST_UTIL = new HBaseTestingUtility();
private Configuration conf;
private String dir;
@Rule
public TestName name = new TestName();
// Test names
protected TableName tableName;
@Before
public void setup() throws IOException {
conf = TEST_UTIL.getConfiguration();
dir = TEST_UTIL.getDataTestDir("TestHRegion").toString();
tableName = TableName.valueOf(name.getMethodName());
}
@Test
public void testWALDurability() throws IOException {
class CustomFSLog extends FSHLog {
private Boolean syncFlag;
public CustomFSLog(FileSystem fs, Path root, String logDir, Configuration conf)
throws IOException {
super(fs, root, logDir, conf);
}
@Override
public void sync(boolean forceSync) throws IOException {
syncFlag = forceSync;
super.sync(forceSync);
}
@Override
public void sync(long txid, boolean forceSync) throws IOException {
syncFlag = forceSync;
super.sync(txid, forceSync);
}
private void resetSyncFlag() {
this.syncFlag = null;
}
}
// global hbase.wal.hsync false, no override in put call - hflush
conf.set(HRegion.WAL_HSYNC_CONF_KEY, "false");
FileSystem fs = FileSystem.get(conf);
Path rootDir = new Path(dir + getName());
CustomFSLog customFSLog = new CustomFSLog(fs, rootDir, getName(), conf);
HRegion region = initHRegion(tableName, null, null, customFSLog);
byte[] bytes = Bytes.toBytes(getName());
Put put = new Put(bytes);
put.addColumn(COLUMN_FAMILY_BYTES, Bytes.toBytes("1"), bytes);
customFSLog.resetSyncFlag();
assertNull(customFSLog.syncFlag);
region.put(put);
assertEquals(customFSLog.syncFlag, false);
// global hbase.wal.hsync true, no override in put call
conf.set(HRegion.WAL_HSYNC_CONF_KEY, "true");
fs = FileSystem.get(conf);
customFSLog = new CustomFSLog(fs, rootDir, getName(), conf);
region = initHRegion(tableName, null, null, customFSLog);
customFSLog.resetSyncFlag();
assertNull(customFSLog.syncFlag);
region.put(put);
assertEquals(customFSLog.syncFlag, true);
// global hbase.wal.hsync true, durability set in put call - fsync
put.setDurability(Durability.FSYNC_WAL);
customFSLog.resetSyncFlag();
assertNull(customFSLog.syncFlag);
region.put(put);
assertEquals(customFSLog.syncFlag, true);
// global hbase.wal.hsync true, durability set in put call - sync
put = new Put(bytes);
put.addColumn(COLUMN_FAMILY_BYTES, Bytes.toBytes("1"), bytes);
put.setDurability(Durability.SYNC_WAL);
customFSLog.resetSyncFlag();
assertNull(customFSLog.syncFlag);
region.put(put);
assertEquals(customFSLog.syncFlag, false);
HBaseTestingUtility.closeRegionAndWAL(region);
}
private String getName() {
return name.getMethodName();
}
/**
* @return A region on which you must call {@link HBaseTestingUtility#closeRegionAndWAL(HRegion)}
* when done.
*/
public static HRegion initHRegion(TableName tableName, byte[] startKey, byte[] stopKey, WAL wal)
throws IOException {
ChunkCreator.initialize(MemStoreLABImpl.CHUNK_SIZE_DEFAULT, false, 0, 0, 0, null);
return TEST_UTIL.createLocalHRegion(tableName, startKey, stopKey, false, Durability.USE_DEFAULT,
wal, COLUMN_FAMILY_BYTES);
}
}

2
hbase-server/src/test/java/org/apache/hadoop/hbase/replication/TestReplicationSource.java
View File

@ -122,7 +122,7 @@ public class TestReplicationSource {
WALKeyImpl key = new WALKeyImpl(b, TableName.valueOf(b), 0, 0,
HConstants.DEFAULT_CLUSTER_ID);
writer.append(new WAL.Entry(key, edit));
writer.sync();
writer.sync(false);
}
writer.close();

2
hbase-server/src/test/java/org/apache/hadoop/hbase/replication/TestSerialReplication.java
View File

@ -99,7 +99,7 @@ public class TestSerialReplication {
for (Entry entry : replicateContext.getEntries()) {
WRITER.append(entry);
}
WRITER.sync();
WRITER.sync(false);
} catch (IOException e) {
throw new UncheckedIOException(e);
}

7
hbase-server/src/test/java/org/apache/hadoop/hbase/wal/FaultyFSLog.java
View File

@ -51,10 +51,15 @@ public class FaultyFSLog extends FSHLog {
@Override
public void sync(long txid) throws IOException {
sync(txid, false);
}
@Override
public void sync(long txid, boolean forceSync) throws IOException {
if (this.ft == FailureType.SYNC) {
throw new IOException("sync");
}
super.sync(txid);
super.sync(txid, forceSync);
}
@Override

4
hbase-server/src/test/java/org/apache/hadoop/hbase/wal/IOTestProvider.java
View File

@ -266,9 +266,9 @@ public class IOTestProvider implements WALProvider {
}
@Override
public void sync() throws IOException {
public void sync(boolean forceSync) throws IOException {
if (doSyncs) {
super.sync();
super.sync(forceSync);
}
}
}

6
hbase-server/src/test/java/org/apache/hadoop/hbase/wal/TestWALSplit.java
View File

@ -1359,7 +1359,7 @@ public class TestWALSplit {
WALKeyImpl key = new WALKeyImpl(hri.getEncodedNameAsBytes(), TABLE_NAME, 1,
EnvironmentEdgeManager.currentTime(), HConstants.DEFAULT_CLUSTER_ID);
w.append(new Entry(key, edit));
w.sync();
w.sync(false);
}
private static void appendRegionEvent(Writer w, String region) throws IOException {
@ -1377,7 +1377,7 @@ public class TestWALSplit {
HConstants.DEFAULT_CLUSTER_ID);
w.append(
new Entry(walKey, new WALEdit().add(kv)));
w.sync();
w.sync(false);
}
public static long appendEntry(Writer writer, TableName table, byte[] region,
@ -1387,7 +1387,7 @@ public class TestWALSplit {
LOG.info(Thread.currentThread().getName() + " append");
writer.append(createTestEntry(table, region, row, family, qualifier, value, seq));
LOG.info(Thread.currentThread().getName() + " sync");
writer.sync();
writer.sync(false);
return seq;
}