刷盘机制支持同步刷盘和异步刷盘。为了了解其具体事项,我们以Commitlog的存储为例来说明RocketMQ是如何进行磁盘读写。
Comitlog#putMessage 首先,主要是将消息写入到MappedFile,内存映射文件。然后根据刷盘策略刷写到磁盘,入口:
CommitLog#putMessage handleDiskFlush
CommitLog#handleDiskFlush
-
public void handleDiskFlush(AppendMessageResult result, PutMessageResult putMessageResult, MessageExt messageExt) { // @1
-
// Synchronization flush
-
if (FlushDiskType.SYNC_FLUSH == this.defaultMessageStore.getMessageStoreConfig().getFlushDiskType()) { // @2
-
final GroupCommitService service = (GroupCommitService) this.flushCommitLogService;
-
if (messageExt.isWaitStoreMsgOK()) {
-
GroupCommitRequest request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes());
-
service.putRequest(request);
-
boolean flushOK = request.waitForFlush(this.defaultMessageStore.getMessageStoreConfig().getSyncFlushTimeout());
-
if (!flushOK) {
-
log.error("do groupcommit, wait for flush failed, topic: " + messageExt.getTopic() + " tags: " + messageExt.getTags()
-
+ " client address: " + messageExt.getBornHostString());
-
putMessageResult.setPutMessageStatus(PutMessageStatus.FLUSH_DISK_TIMEOUT);
-
}
-
} else {
-
service.wakeup();
-
}
-
}
-
// Asynchronous flush
-
else { // @3
-
if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) {
-
flushCommitLogService.wakeup();
-
} else {
-
commitLogService.wakeup();
-
}
-
}
-
}
代码@1 参数详解
AppendMessageResult result 写入到MappedFile(内存映射文件中,bytebuffer)中的结果,具体属性包含:
wroteOffset : 下一个写入的偏移量
wroteBytes : 写入字节总长度
msgId : 消息id
storeTimestamp : 消息存储时间,也就是写入到MappedFile中的时间
logicOffset : 逻辑的consumeque 偏移量
pagecacheRT : 写入到MappedByteBuffer(将消息内容写入到内存映射文件中的时长)
代码@2 同步刷盘
代码@3 异步刷盘
1、同步刷盘线程
同步刷盘机制,核心实现类 CommitLog#GroupCommitService
同步刷盘核心类,竟然是一个线程,出乎我的意料。
1.1 核心属性
private volatile List<GroupCommitRequest> requestsWrite = new ArrayList<GroupCommitRequest>();
private volatile List<GroupCommitRequest> requestsRead = new ArrayList<GroupCommitRequest>();
requestsWrite : 写队列,主要用于向该线程添加刷盘任务
requestsRead :读队列,主要用于执行特定的刷盘任务,这是是GroupCommitService 设计的一个亮点,把读写分离,每处理完requestsRead中的任务,就交换这两个队列。
1.2 对外方法putRequest,添加刷盘任务
-
public synchronized void putRequest(final GroupCommitRequest request) {
-
synchronized (this.requestsWrite) {
-
this.requestsWrite.add(request);
-
}
-
if (hasNotified.compareAndSet(false, true)) {
-
waitPoint.countDown(); // notify
-
}
-
}
该方法是很简单,就是将GroupCommitRequest刷盘任务放入到requestWrite中,就返回了,但是这个类是处理同步刷盘的,那调用方什么时候才能知道该刷盘任务已经执行了呢?
不然能说是同步刷盘呢?这又是这个类另外一个设计亮点。为了解开这个疑点,首先看一下调用方法:
GroupCommitRequest request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes());
service.putRequest(request);
boolean flushOK = request.waitForFlush(this.defaultMessageStore.getMessageStoreConfig().getSyncFlushTimeout());
原来奥秘在这里,放入后,request.waitForFlush,类似于Future模式,在这方法里进行阻塞等待。
this.countDownLatch.await(timeout, TimeUnit.MILLISECONDS),默认同步刷盘超时时间为5s,那就不需要怀疑了,刷盘后,肯定会调用countDownLatch.countDown()
GroupCommitRequest 具体类的工作机制就不细说了,其刷盘将调用的方法为:CommitLog.this.mappedFileQueue.flush(0);
在进入具体刷盘逻辑之前,我们再看下异步刷盘线程的实现。
2、异步刷盘线程
-
if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) {
-
flushCommitLogService.wakeup();
-
} else {
-
commitLogService.wakeup();
-
}
-
public boolean isTransientStorePoolEnable() {
-
return transientStorePoolEnable && FlushDiskType.ASYNC_FLUSH == getFlushDiskType()
-
&& BrokerRole.SLAVE != getBrokerRole();
-
}
什么是transientStorePoolEnable ,这个只能从FlushRealTimeService 与 CommitRealTimeService 区别
2.1 FlushRealTimeService 实现机制
-
class FlushRealTimeService extends FlushCommitLogService {
-
private long lastFlushTimestamp = 0;
-
private long printTimes = 0;
-
public void run() {
-
CommitLog.log.info(this.getServiceName() + " service started");
-
while (!this.isStopped()) {
-
boolean flushCommitLogTimed = CommitLog.this.defaultMessageStore.getMessageStoreConfig().isFlushCommitLogTimed(); // @1
-
int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushIntervalCommitLog(); // @2
-
int flushPhysicQueueLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogLeastPages(); // @3
-
int flushPhysicQueueThoroughInterval =
-
CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogThoroughInterval(); // @4
-
boolean printFlushProgress = false;
-
// Print flush progress
-
long currentTimeMillis = System.currentTimeMillis();
-
if (currentTimeMillis >= (this.lastFlushTimestamp + flushPhysicQueueThoroughInterval)) {
-
this.lastFlushTimestamp = currentTimeMillis;
-
flushPhysicQueueLeastPages = 0;
-
printFlushProgress = (printTimes++ % 10) == 0;
-
}
-
try {
-
if (flushCommitLogTimed) {
-
Thread.sleep(interval);
-
} else {
-
this.waitForRunning(interval);
-
}
-
if (printFlushProgress) {
-
this.printFlushProgress();
-
}
-
long begin = System.currentTimeMillis();
-
CommitLog.this.mappedFileQueue.flush(flushPhysicQueueLeastPages);
-
long storeTimestamp = CommitLog.this.mappedFileQueue.getStoreTimestamp();
-
if (storeTimestamp > 0) {
-
CommitLog.this.defaultMessageStore.getStoreCheckpoint().setPhysicMsgTimestamp(storeTimestamp);
-
}
-
long past = System.currentTimeMillis() - begin;
-
if (past > 500) {
-
log.info("Flush data to disk costs {} ms", past);
-
}
-
} catch (Throwable e) {
-
CommitLog.log.warn(this.getServiceName() + " service has exception. ", e);
-
this.printFlushProgress();
-
}
-
}
-
// Normal shutdown, to ensure that all the flush before exit
-
boolean result = false;
-
for (int i = 0; i < RETRY_TIMES_OVER && !result; i++) {
-
result = CommitLog.this.mappedFileQueue.flush(0);
-
CommitLog.log.info(this.getServiceName() + " service shutdown, retry " + (i + 1) + " times " + (result ? "OK" : "Not OK"));
-
}
-
this.printFlushProgress();
-
CommitLog.log.info(this.getServiceName() + " service end");
-
}
代码@1:flushCommitLogTimed 这个主要是等待方法,如果为true,则使用Thread.sleep,如果是false使用waitForRunning
代码@2:interval :获取刷盘的间隔时间
代码@3:flushPhysicQueueLeastPages 每次刷盘最少需要刷新的页,(如果少于,是不是可以不放弃本次刷盘操作
代码@4:flushPhysicQueueThoroughInterval 如果上次刷新的时间+该值 小于当前时间,则改变flushPhysicQueueLeastPages =0,并每10次输出异常刷新进度。
代码@5:CommitLog.this.mappedFileQueue.flush(flushPhysicQueueLeastPages); 调用刷盘操作
代码@6:设置检测点的StoreCheckpoint 的physicMsgTimestamp(commitlog文件的检测点,也就是记录最新刷盘的时间戳)
暂时不深入,在本节之后详细分析刷盘机制。
2.2 CommitRealTimeService
-
public void run() {
-
CommitLog.log.info(this.getServiceName() + " service started");
-
while (!this.isStopped()) {
-
int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitIntervalCommitLog(); // @1
-
int commitDataLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogLeastPages(); // @2
-
int commitDataThoroughInterval =
-
CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogThoroughInterval(); // @3
-
long begin = System.currentTimeMillis();
-
if (begin >= (this.lastCommitTimestamp + commitDataThoroughInterval)) {
-
this.lastCommitTimestamp = begin;
-
commitDataLeastPages = 0;
-
}
-
try {
-
boolean result = CommitLog.this.mappedFileQueue.commit(commitDataLeastPages);
-
long end = System.currentTimeMillis();
-
if (!result) {
-
this.lastCommitTimestamp = end; // result = false means some data committed.
-
//now wake up flush thread.
-
flushCommitLogService.wakeup();
-
}
-
if (end - begin > 500) {
-
log.info("Commit data to file costs {} ms", end - begin);
-
}
-
this.waitForRunning(interval);
-
} catch (Throwable e) {
-
CommitLog.log.error(this.getServiceName() + " service has exception. ", e);
-
}
-
}
-
boolean result = false;
-
for (int i = 0; i < RETRY_TIMES_OVER && !result; i++) {
-
result = CommitLog.this.mappedFileQueue.commit(0);
-
CommitLog.log.info(this.getServiceName() + " service shutdown, retry " + (i + 1) + " times " + (result ? "OK" : "Not OK"));
-
}
-
CommitLog.log.info(this.getServiceName() + " service end");
-
}
-
}
代码@1:interval CommitRealTimeService 执行间隔
代码@2:commitDataLeastPages :每次commit最少的页数
代码@3:上上面的对应,,CommitRealTimeService与FlushRealTimeService 不同之处,是调用的方法不一样,
FlushRealTimeService 调用mappedFileQueue.flush,而CommitRealTimeService调用commit方法。
行文至此,我们只是了解异步刷盘,同步刷盘去线程的实现方式,接下来,是时候进入到刷盘具体逻辑,也就是Commitlog mappedFileQueue
3、刷盘机制实现
具体实现类:MappedFileQueue
3.1 核心属性与构造方法
-
private static final int DELETE_FILES_BATCH_MAX = 10;
-
private final String storePath;
-
private final int mappedFileSize;
-
private final CopyOnWriteArrayList<MappedFile> mappedFiles = new CopyOnWriteArrayList<MappedFile>();
-
private final AllocateMappedFileService allocateMappedFileService;
-
private long flushedWhere = 0;
-
private long committedWhere = 0;
-
private volatile long storeTimestamp = 0;
-
public MappedFileQueue(final String storePath, int mappedFileSize,
-
AllocateMappedFileService allocateMappedFileService) {
-
this.storePath = storePath;
-
this.mappedFileSize = mappedFileSize;
-
this.allocateMappedFileService = allocateMappedFileService;
-
}
MappedFileQueue 就是MappedFile的队列,也就是MappedFile的容器。
storePath:文件存储路径
mappedFileSize:单个MappedFile文件长度
mappedFiles :mappedFile集合
allocateMappedFileService: 创建MappedFileService
flushedWhere :刷盘位置
committedWhere :commit位置
我们先看一下MappedFileQueue在什么时候创建:
我们以commitlog为例:
this.mappedFileQueue = new MappedFileQueue(defaultMessageStore.getMessageStoreConfig().getStorePathCommitLog(),
defaultMessageStore.getMessageStoreConfig().getMapedFileSizeCommitLog(), defaultMessageStore.getAllocateMappedFileService());
其中allocateMappedFileService 为AllocateMappedFileService
MappedFileQueue 就是MappedFile的队列,也就是MappedFile的容器。
storePath:文件存储路径
mappedFileSize:单个MappedFile文件长度
mappedFiles :mappedFile集合
allocateMappedFileService: 创建MappedFileService
flushedWhere :刷盘位置
committedWhere :commit位置
3.2 核心方法:load
-
public boolean load() {
-
File dir = new File(this.storePath);
-
File[] files = dir.listFiles();
-
if (files != null) {
-
// ascending order
-
Arrays.sort(files);
-
for (File file : files) {
-
if (file.length() != this.mappedFileSize) {
-
log.warn(file + "\t" + file.length()
-
+ " length not matched message store config value, ignore it");
-
return true;
-
}
-
try {
-
MappedFile mappedFile = new MappedFile(file.getPath(), mappedFileSize);
-
mappedFile.setWrotePosition(this.mappedFileSize);
-
mappedFile.setFlushedPosition(this.mappedFileSize);
-
mappedFile.setCommittedPosition(this.mappedFileSize);
-
this.mappedFiles.add(mappedFile);
-
log.info("load " + file.getPath() + " OK");
-
} catch (IOException e) {
-
log.error("load file " + file + " error", e);
-
return false;
-
}
-
}
-
}
-
return true;
-
}
// 该方法主要是按顺序,创建MappedFile,此时这个时候关注一下,初始化时wrotePosition,flushedPosition,committedPosition全设置为最大值,这要怎么玩呢?是否还记得启动时需要恢复commitlog,consume,index文件,(recover)方法,在删除无效文件时,会重置上述值
接下来,我们先梳理一下目前刷盘出现的关键属性,然后进入到刷盘机制的世界中来:
1、MappedFileQueue 与MappedFile的关系
可以这样认为,MappedFile代表一个个物理文件,而MappedFileQueue代表由一个个MappedFile组成的一个连续逻辑的大文件。
并且每一个MappedFile的命名已该文件在整个文件序列中的偏移量来表示。
2、MappedFileQueue
flushedWhere: 整个刷新的偏移量,针对该MappedFileQueue
committedWhere:当前提交的偏移量,针对该MappedFileQueue commit与flush的区别?
3、MappedFile
wrotePosition :当前待写入位置
committedPosition 提交位置
flushedPosition 刷新位置 应该是 commitedPosition <= flushedPosition
接下来,主要来看MappedFileQueue comit flush方法
3.3 MappedFileQueue #commit
-
public boolean commit(final int commitLeastPages) {
-
boolean result = true;
-
MappedFile mappedFile = this.findMappedFileByOffset(this.committedWhere, false); // @1
-
if (mappedFile != null) {
-
int offset = mappedFile.commit(commitLeastPages); // @2
-
long where = mappedFile.getFileFromOffset() + offset; // @3
-
result = where == this.committedWhere; // @4
-
this.committedWhere = where; // @5
-
}
-
return result;
-
}
代码@1:根据committedWhere 找到具体的MappedFile文件
代码@2:调用MappedFile的commit函数
代码@3,mappedFile返回的应该是当前commit的偏移量,加上该文件开始的偏移,,表示MappedFileQueue当前的提交偏移量
代码@4:如果result = true,则可以认为MappedFile#commit 本次并没有执行commit操作
代码@5,更新当前的ccomitedWhere指针。
接下来继续查看MappedFile#commit的实现:
MappedFile#commit()
-
public int commit(final int commitLeastPages) { // @1
-
if (writeBuffer == null) { // @2
-
//no need to commit data to file channel, so just regard wrotePosition as committedPosition.
-
return this.wrotePosition.get();
-
}
-
if (this.isAbleToCommit(commitLeastPages)) { // @3
-
if (this.hold()) {
-
commit0(commitLeastPages);
-
this.release();
-
} else {
-
log.warn("in commit, hold failed, commit offset = " + this.committedPosition.get());
-
}
-
}
-
// All dirty data has been committed to FileChannel.
-
if (writeBuffer != null && this.transientStorePool != null && this.fileSize == this.committedPosition.get()) {
-
this.transientStorePool.returnBuffer(writeBuffer);
-
this.writeBuffer = null;
-
}
-
return this.committedPosition.get();
-
}
代码@1:参数,commitLeastPages 至少提交的页数,如果当前需要提交的数据所占的页数小于commitLeastPages ,则不执行本次提交操作
代码@2:如果writeBuffer 等于null,则表示IO操作都是直接基于FileChannel,所有,此时返回当前可写的位置,作为committedPosition即可,这里应该就有点commit是个啥意思了,
如果数据先写入到writeBuffer中,则需要提交到FileChannel(MappedByteBuffer mappedByteBuffer)
代码@3:判断是否可以执行提交操作
-
protected boolean isAbleToCommit(final int commitLeastPages) {
-
int flush = this.committedPosition.get();
-
int write = this.wrotePosition.get();
-
if (this.isFull()) {
-
return true;
-
}
-
if (commitLeastPages > 0) {
-
return ((write / OS_PAGE_SIZE) - (flush / OS_PAGE_SIZE)) >= commitLeastPages;
-
}
-
return write > flush;
-
}
从代码可以看出
@1 如果文件写满(this.fileSize == this.wrotePosition.get()) 则可以执行commit
@2 如果有最小提交页数要求,则(当前写入位置/ pagesize(4k) - 当前flush位置/pagesize(4k) 大于commitLeastPages时,再提交。
@3,如果没有最新提交页数要求,则只有当前写入位置大于flush,则可提交。
代码@4:执行具体的提交操作
-
protected void commit0(final int commitLeastPages) {
-
int writePos = this.wrotePosition.get();
-
int lastCommittedPosition = this.committedPosition.get();
-
if (writePos - this.committedPosition.get() > 0) {
-
try {
-
ByteBuffer byteBuffer = writeBuffer.slice(); // @1
-
byteBuffer.position(lastCommittedPosition);
-
byteBuffer.limit(writePos);
-
this.fileChannel.position(lastCommittedPosition);
-
this.fileChannel.write(byteBuffer); // @2
-
this.committedPosition.set(writePos); // @3
-
} catch (Throwable e) {
-
log.error("Error occurred when commit data to FileChannel.", e);
-
}
-
}
-
}
代码@1,这里使用slice方法,主要是用的同一片内存空间,但单独的指针。
代码@2:将bytebuf当前 上一次commitedPosition + 当前写位置这些数据全部写入到FileChannel中,commit的左右原来是这要的,是将writeBuffer中的数据写入到FileChannel中
代码@3:更新committedPosition的位置。
讲到这里,commit 的作用就非常明白了,为了加深理解,该是来理解MappedFile几个核心属性的时候了。
protected int fileSize; // 文件的大小
protected FileChannel fileChannel; // 文件通道
/**
* Message will put to here first, and then reput to FileChannel if writeBuffer is not null.
*/
protected ByteBuffer writeBuffer = null; // 如果不为空,内容写写入到writeBuffer,然后再重新放入到FileChannel中,这个重新放入,其实就是commit操作。
protected TransientStorePool transientStorePool = null; // 临时存储,只有pool不为空,wrtieBuffer才不会为空,也就是MessageStoreConfig中transientStorePoolEnable设置为true时
// 才会生效,也是writeBuffer的容器。也就是writeBuffer其实就是堆内内存,如果transientStorePoolEnable为true,消息是先直
// 接放入到堆内存中,然后定时commit到堆外内存(FileChannel,MappedByteBuffer)中,再定时flush.
private long fileFromOffset; // 文件初始偏移量(在整个MappedFile链表中的逻辑偏移量)
private File file; // 物理文件
private MappedByteBuffer mappedByteBuffer; // mappedByteBufer,FileChanel的内存映射。
如果启用了MessageStoreConfig的transientStorePoolEnable=true,消息在追加时,先放入到writeBuffer中,然后定时commit到FileChannel,,然后定时flush,如果transientStorePoolEnable=false(默认)则消息追加时,直接存入MappedByteBuffer中,然后定时flush ,【备注,说的是异步刷盘,如果是同步刷盘】,应该是直接调用flush方法。
接下来我们再看一下flush方法,其实基本明了了,就是调用FileChannel的force()方法。
3.4 MappedFileQueue#flush MappedFile#flush
-
public boolean flush(final int flushLeastPages) {
-
boolean result = true;
-
MappedFile mappedFile = this.findMappedFileByOffset(this.flushedWhere, false);
-
if (mappedFile != null) {
-
long tmpTimeStamp = mappedFile.getStoreTimestamp();
-
int offset = mappedFile.flush(flushLeastPages);
-
long where = mappedFile.getFileFromOffset() + offset;
-
result = where == this.flushedWhere;
-
this.flushedWhere = where;
-
if (0 == flushLeastPages) {
-
this.storeTimestamp = tmpTimeStamp;
-
}
-
}
-
return result;
-
}
-
/**
-
* @return The current flushed position
-
*/
-
public int flush(final int flushLeastPages) {
-
if (this.isAbleToFlush(flushLeastPages)) {
-
if (this.hold()) {
-
int value = getReadPosition();
-
try {
-
//We only append data to fileChannel or mappedByteBuffer, never both.
-
if (writeBuffer != null || this.fileChannel.position() != 0) {
-
this.fileChannel.force(false);
-
} else {
-
this.mappedByteBuffer.force();
-
}
-
} catch (Throwable e) {
-
log.error("Error occurred when force data to disk.", e);
-
}
-
this.flushedPosition.set(value);
-
this.release();
-
} else {
-
log.warn("in flush, hold failed, flush offset = " + this.flushedPosition.get());
-
this.flushedPosition.set(getReadPosition());
-
}
-
}
-
return this.getFlushedPosition();
-
}
具体代码很好理解,就不一一分析了。
RocketMQ的刷盘机制就介绍到这,我们再简单做个总结
先讲一下RocketMQ的存储设计亮点:(以CommitLog为例)
单个commitlog文件,默认大小为1G,由多个commitlog文件来存储所有的消息,commitlog文件的命名以该文件在整个commitlog中的偏移量来命名,举例如下,
例如一个commitlog文件,1024个字节,
第一个文件: 00000000000000000000
第二个文件: 00000000000000001024
MappedFile封装一个一个的CommitLog文件,而MappedFileQueue就是封装的就是一个逻辑的commitlog文件。mappedFile队列,从小到大排列。
使用内存映射机制,MappedByteBuffer,具体封装类为MappedFile。
1、同步刷盘 每次发送消息,消息都直接存储在FileChannel中,使用的是(MapFile的mappdByteBuffer),然后直接调用force()方法刷写到磁盘,等到force刷盘成功后,再返回给调用发(GroupCommitRequest#waitForFlush)就是其同步调用的实现。
2、异步刷盘
分为两种情况,是否开启堆内存缓存池,具体配置参数:MessageStoreConfig#transientStorePoolEnable
transientStorePoolEnable=true
消息在追加时,先放入到writeBuffer中,然后定时commit到FileChannel,,然后定时flush,
transientStorePoolEnable=false(默认)
消息追加时,直接存入MappedByteBuffer中,然后定时flush ,【备注,说的是异步刷盘,如果是同步刷盘】,应该是直接调用flush方法。
MappedFile 重要的指针
wrotePosition:当前写入的指针
committedPosition : 上一次提交的指针 (transientStorePoolEnable=true时有效)
flushedPosition : 上一次flush的指针
OS_PAGE_SIZE = 1024 * 4 : 一页大小,4K
flushedPosition <= committedPosition <= wrotePosition <= fileSIze
来源:oschina
链接:https://my.oschina.net/u/866802/blog/1928117