一、刷盘机制       

        刷盘策略在不同时间进行刷写磁盘。RocketMQ的存储是基于JDK NIO的内存映射机制(MappedByteBuffer)的，消息存储首先将消息追加到内存，再根据配置的刷写磁盘

        同步刷盘表示消息追加到内存后，立即将数据刷写到文件系统中。代码的调用链如下：

        submitFlushRequest代码源码如下：

public CompletableFuture<PutMessageStatus> submitFlushRequest(AppendMessageResult result, MessageExt messageExt) {
        // Synchronization flush
        if (FlushDiskType.SYNC_FLUSH == this.defaultMessageStore.getMessageStoreConfig().getFlushDiskType()) {
            final GroupCommitService service = (GroupCommitService) this.flushCommitLogService;
            if (messageExt.isWaitStoreMsgOK()) {
                GroupCommitRequest request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes(),
                        this