我们都知道RocketMQ的消息是持久化到文件的，具体的消息的刷盘策略是什么，是发送一条消息就直接持久化到文件中吗？作为一款高性能的消息中间件这样做肯定不行，至少性能上不允许这样操作，那么具体策略是啥我们具体分析下。

1、刷盘策略

RocketMQ提供了两种刷盘策略同步刷盘、异步刷盘

同步刷盘：在消息到达MQ后，RocketMQ需要将数据持久化，同步刷盘是指数据到达内存之后，必须刷到commitlog日志之后才算成功，然后返回producer数据已经发送成功。

异步刷盘：，同步刷盘是指数据到达内存之后,返回producer说数据已经发送成功。，然后再写入commitlog日志。

复制方式	优点	缺点	适应场景
同步刷盘	保证了消息不丢失	吞吐率相对于异步刷盘要低	消息可靠性要求较高的场景
异步刷盘	系统的吞吐量提高	系统断电等异常时会有部分丢失	对应吞吐量要求较高的场景

下面我们从源码的角度分析其实现的逻辑

image

2、同步刷盘

CommitLog.putMessage()方法中的刷盘的核心方法handleDiskFlush()

public void handleDiskFlush(AppendMessageResult result, PutMessageResult putMessageResult, MessageExt messageExt) {
    // Synchronization flush  同步刷盘
    if (FlushDiskType.SYNC_FLUSH == this.defaultMessageStore.getMessageStoreConfig().getFlushDiskType()) {
        final GroupCommitService service = (GroupCommitService) this.flushCommitLogService;
        //客户端确认要等待刷盘成功
        if (messageExt.isWaitStoreMsgOK()) {
            //封装刷盘请求对象 nextoffset : 当前内存写的位置 + 本次要写入的字节数
            GroupCommitRequest request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes());
            //添加刷盘请求（后台定时任务进行刷盘，每隔10毫秒批量刷盘。10毫秒中如果有多个请求，则多个请求一块刷盘）
            service.putRequest(request);
            //等待刷盘请求结果（最长等待5秒钟，刷盘成功后马上可以获取结果。）
            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();
        }
    }else {// Asynchronous flush 异步刷盘
        if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) {
            //唤醒FlushRealTimeService服务线程
            flushCommitLogService.wakeup();
        } else {
            //唤醒CommitRealTimeService服务线程
            commitLogService.wakeup();
        }
    }
}

查看同步刷盘的核心类GroupCommitService中的核心属性

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。

我们查看其run()方法

public void run() {
    CommitLog.log.info(this.getServiceName() + " service started");
    while (!this.isStopped()) {
        try {
            //等待通知，如果数据过来，提前结束等待执行onWaitEnd()方法交换读写swapRequests（）
            //刷盘请求的requestsWrite->requestsRead
            this.waitForRunning(10);
            //执行刷盘
            this.doCommit();
        } catch (Exception e) {
            CommitLog.log.warn(this.getServiceName() + " service has exception. ", e);
        }
    }
    //省略代码...
}

waitForRunning方法中执行了swapRequests()方法

private void swapRequests() {
    List<GroupCommitRequest> tmp = this.requestsWrite;
    this.requestsWrite = this.requestsRead;
    this.requestsRead = tmp;
}

GroupCommitService接收到的刷盘请求通过putRequest()方法加入到requestsWrite集合中，swapRequests()方法将requestsWrite请求集合交换到requestsRead集合中供刷盘使用，我们重点查看doCommit()方法

private void doCommit() {
    synchronized (this.requestsRead) {
        if (!this.requestsRead.isEmpty()) {
            //循环每一个刷盘请求
            for (GroupCommitRequest req : this.requestsRead) {
                // There may be a message in the next file, so a maximum of
                // two times the flush
                boolean flushOK = false;
                for (int i = 0; i < 2 && !flushOK; i++) {
                    //判断是否已经刷盘过了，刷盘的位置和当前消息下次刷盘需要的位置比较
                    flushOK = CommitLog.this.mappedFileQueue.getFlushedWhere() >= req.getNextOffset();
                    if (!flushOK) {
                        //0代码立刻刷盘，不管缓存中消息有多少
                        CommitLog.this.mappedFileQueue.flush(0);
                    }
                }
                //返回刷盘的结果
                req.wakeupCustomer(flushOK);
            }
            long storeTimestamp = CommitLog.this.mappedFileQueue.getStoreTimestamp();
            //设置刷盘的时间点
            if (storeTimestamp > 0) {
                CommitLog.this.defaultMessageStore.getStoreCheckpoint().setPhysicMsgTimestamp(storeTimestamp);
            }
            //清空requestsRead对象
            this.requestsRead.clear();
        } else {
            // Because of individual messages is set to not sync flush, it
            // will come to this process
            CommitLog.this.mappedFileQueue.flush(0);
        }
    }
}

mappedFileQueue.flush(0)立刻刷盘

public boolean flush(final int flushLeastPages) {
    boolean result = true;
    MappedFile mappedFile = this.findMappedFileByOffset(this.flushedWhere, this.flushedWhere == 0);
    if (mappedFile != null) {
        long tmpTimeStamp = mappedFile.getStoreTimestamp();
        //刷盘，返回刷写到磁盘指针
        int offset = mappedFile.flush(flushLeastPages);
        //计算当前的刷盘指针，之前的所有数据已经持久化到磁盘中
        long where = mappedFile.getFileFromOffset() + offset;
        result = where == this.flushedWhere;
        this.flushedWhere = where;
        if (0 == flushLeastPages) {
            this.storeTimestamp = tmpTimeStamp;
        }
    }
    return result;
}

mappedFile.flush(0);保证立刻刷盘后面异步刷盘时也会调用mappedFile.flush()方法

3、异步刷盘

if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) {
    //唤醒FlushRealTimeService服务线程
    flushCommitLogService.wakeup();
} else {
    //唤醒CommitRealTimeService服务线程
    commitLogService.wakeup();
}

我们发现异步刷盘的时候有两种方式，一种是堆外内存池开启时启动CommitRealTimeService服务线程，另一个是默认执行的FlushRealTimeService服务线程进行刷盘操作，关于TransientStorePoolEnable在《RocketMQ内存映射》章节中的“创建映射文件MappedFile”中有介绍

image

​ 图3-1

1、FlushRealTimeService

查看其run()方法

public void run() {
    CommitLog.log.info(this.getServiceName() + " service started");
    while (!this.isStopped()) {
        // 每次刷盘的间隔时间，默认 200ms
        int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitIntervalCommitLog();
        // 每次commit最少的页数 默认4页
        int commitDataLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogLeastPages();
        // 如果上次刷新的时间+该值 小于当前时间，则改变flushPhysicQueueLeastPages =0 默认为200
        int commitDataThoroughInterval =
            CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogThoroughInterval();

        long begin = System.currentTimeMillis();
        //距离上一次刷盘时间超过200ms则立刻刷盘，commit最少的页数置为0
        if (begin >= (this.lastCommitTimestamp + commitDataThoroughInterval)) {
            this.lastCommitTimestamp = begin;
            commitDataLeastPages = 0;
        }
        try {
            //刷盘
            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");
    }
}

这种方式和同步刷盘一样就是mappedFileQueue.commit(commitDataLeastPages)参数有限制，数据达到一定量的时候才进行刷盘操作提高数据的刷盘性能。

2、CommitRealTimeService

查看其run()方法

public void run() {
    CommitLog.log.info(this.getServiceName() + " service started");
    while (!this.isStopped()) {
        // 每次刷盘的间隔时间，默认 200ms
        int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitIntervalCommitLog();
        // 每次commit最少的页数 默认4页
        int commitDataLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogLeastPages();
        // 如果上次刷新的时间+该值 小于当前时间，则改变flushPhysicQueueLeastPages =0 默认为200
        int commitDataThoroughInterval =
            CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogThoroughInterval();

        long begin = System.currentTimeMillis();
        //距离上一次刷盘时间超过200ms则立刻刷盘，commit最少的页数置为0
        if (begin >= (this.lastCommitTimestamp + commitDataThoroughInterval)) {
            this.lastCommitTimestamp = begin;
            commitDataLeastPages = 0;
        }
        try {
            //刷盘
            boolean result = CommitLog.this.mappedFileQueue.commit(commitDataLeastPages);
            long end = System.currentTimeMillis();
            //返回的是false说明数据已经commit到了fileChannel中
            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");
    }
}

我们发现其刷盘方法不一样mappedFileQueue.commit()调用MappedFile.commit()方法

public int commit(final int commitLeastPages) {
    if (writeBuffer == null) {
        //no need to commit data to file channel, so just regard wrotePosition as committedPosition.
        return this.wrotePosition.get();
    }
    //如果提交的数据不满commitLeastPages则不执行本次的提交，待下一次提交
    if (this.isAbleToCommit(commitLeastPages)) {
        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();
}

查看其核心刷盘方法

protected void commit0(final int commitLeastPages) {
    int writePos = this.wrotePosition.get();
    int lastCommittedPosition = this.committedPosition.get();
    if (writePos - this.committedPosition.get() > 0) {
        try {
            //创建writeBuffer的共享缓存区
            ByteBuffer byteBuffer = writeBuffer.slice();
            //将指针回退到上一次提交的位置
            byteBuffer.position(lastCommittedPosition);
            //设置limit为writePos
            byteBuffer.limit(writePos);
            this.fileChannel.position(lastCommittedPosition);
            //将committedPosition指针到wrotePosition的数据复制（写入）到fileChannel中
            this.fileChannel.write(byteBuffer);
            //更新committedPosition指针为writePos
            this.committedPosition.set(writePos);
        } catch (Throwable e) {
            log.error("Error occurred when commit data to FileChannel.", e);
        }
    }
}

commit0（）只是将缓存数据加入到fileChannel中，我们在CommitRealTimeService.run()方法中看到唤醒flushCommitLogService线程需要将fileChannel中的数据flush到磁盘中，我们发现两种方式都需要走flushCommitLogService.run()方法最后都执行MappedFile.flush(int)

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);
            }
            //设置刷盘后的指针
            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();
}

两种缓存方式走的刷盘逻辑也不同，可以查看“图3-1”两种方式的处理流程图

我们还发现一个方法isAbleToFlush()判断是否需要刷盘

private boolean isAbleToFlush(final int flushLeastPages) {
    int flush = this.flushedPosition.get();
    int write = getReadPosition();
    if (this.isFull()) {
        return true;
    }
    if (flushLeastPages > 0) {
        return ((write / OS_PAGE_SIZE) - (flush / OS_PAGE_SIZE)) >= flushLeastPages;
    }
    return write > flush;
}

同步刷盘时flushLeastPages=0立刻刷盘

异步刷盘时flushLeastPages=4 ,默认是4，需要刷盘的数据达到PageCache的页数4倍时才会刷盘，或者距上一次刷盘时间>=200ms则设置flushLeastPages=0立刻刷盘

同步刷盘时无论消息的大小都立刻刷盘，线程阻塞等待刷盘结果

异步刷盘有两种方式但是其逻辑都是需要刷盘的数据OS_PAGE_SIZE的4倍即（1024 * 4）*4=16k或者距上一次刷盘时间>=200ms时才刷盘，提高数据的刷盘性能