Redis ，又抖了！

24:S 16 Aug 10:32:03.357 * Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.
24:S 16 Aug 10:32:05.067 * Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.
24:S 16 Aug 10:32:39.058 * Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.
24:S 16 Aug 10:33:13.067 * Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.
24:S 16 Aug 10:33:47.064 * Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.
24:S 16 Aug 10:34:21.099 * Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.

-----------函数注释-----------
/* Write the append only file buffer on disk.
*
* Since we are required to write the AOF before replying to the client,
* and the only way the client socket can get a write is entering when the
* the event loop, we accumulate all the AOF writes in a memory
* buffer and write it on disk using this function just before entering
* the event loop again.
*
函数的作用是：将AOF buffer的内容刷到磁盘上，一般情况下在回复客户端响应之前需要将AOF写入，
业务在事件循环结束的时候才能收到已经写入数据的反馈，我们把所有的AOF写入在一个内存buffer中，
然后使用这个函数将它写入磁盘，然后开始进入下一个时间循环

* About the 'force' argument:
*
* When the fsync policy is set to 'everysec' we may delay the flush if there
* is still an fsync() going on in the background thread, since for instance
* on Linux write(2) will be blocked by the background fsync anyway.
* When this happens we remember that there is some aof buffer to be
* flushed ASAP, and will try to do that in the serverCron() function.
*
* However if force is set to 1 we'll write regardless of the background
* fsync. */
---------------函数--------------
#define AOF_WRITE_LOG_ERROR_RATE 30 /* Seconds between errors logging. */
void flushAppendOnlyFile(int force) {
    ssize_t nwritten;
    int sync_in_progress = 0;
    mstime_t latency;

    if (sdslen(server.aof_buf) == 0) return;

    if (server.aof_fsync == AOF_FSYNC_EVERYSEC)
        sync_in_progress = bioPendingJobsOfType(BIO_AOF_FSYNC) != 0;

    if (server.aof_fsync == AOF_FSYNC_EVERYSEC && !force) {
        /* With this append fsync policy we do background fsyncing.
         * If the fsync is still in progress we can try to delay
         * the write for a couple of seconds. */
        if (sync_in_progress) {
            if (server.aof_flush_postponed_start == 0) {
                /* No previous write postponing, remember that we are
                 * postponing the flush and return. */
                 * 前面没有推迟过 write 操作，这里将推迟写操作的时间记录下来
                 * 然后就返回，不执行 write 或者 fsync
                server.aof_flush_postponed_start = server.unixtime;
                return;
            } else if (server.unixtime - server.aof_flush_postponed_start < 2) {
                /* We were already waiting for fsync to finish, but for less
                 * than two seconds this is still ok. Postpone again. */
                 * 如果之前已经因为 fsync 而推迟了 write 操作
                 * 但是推迟的时间不超过 2 秒，那么直接返回
                 * 不执行 write 或者 fsync
                return;
            }
            /* Otherwise fall trough, and go write since we can't wait
             * over two seconds. */
             * 如果后台还有 fsync 在执行，并且 write 已经推迟 >= 2 秒
             * 那么执行写操作（write 将被阻塞）
            server.aof_delayed_fsync++;
            serverLog(LL_NOTICE,"Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.");
        }
    }

# 查看内存的脏页字节大小，设置为0代表由系统自己控制何时调用fsync
sysctl -a | grep vm.dirty_bytes
vm.dirty_bytes = 0

# 修改为一个小的值，例如32MB，达到这个数据量就fsync，让操作系统fsync这个动作更频繁一点，避免单次fsync太多数据，导致阻塞
echo "vm.dirty_bytes=33554432" >> /etc/sysctl.conf