redis.conf中的appendfysnc是对redis性能有重要影响的参数之一。可取三种值：always、everysec和no。
设置为always时，会极大消弱Redis的性能，因为这种模式下每次write后都会调用fsync（Linux为调用fdatasync）。
如果设置为no，则write后不会有fsync调用，由操作系统自动调度刷磁盘，性能是最好的。
everysec为最多每秒调用一次fsync，这种模式性能并不是很糟糕，一般也不会产生毛刺，这归功于Redis引入了BIO线程，所有fsync操作都异步交给了BIO线程。
另外，Redis在处理一条命令时，并不立即调用write写AOF文件，只是将数据写入到AOF buffer（server.aof_buf）中。调用write和命令处理是分开的，Redis只在每次进入epoll_wait之前做write操作。
|
/* 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. * * 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. */ #define AOF_WRITE_LOG_ERROR_RATE 30 /* Seconds between errors logging. */ void flushAppendOnlyFile(int force) { // aofWrite调用write将AOF buffer写入到AOF文件，处理了ENTR，其它没什么 ssize_t nwritten = aofWrite(server.aof_fd,server.aof_buf,sdslen(server.aof_buf)); 。。。。。。 /* Handle the AOF write error. */ if (server.aof_fsync == AOF_FSYNC_ALWAYS) { /* We can‘t recover when the fsync policy is ALWAYS since the * reply for the client is already in the output buffers, and we * have the contract with the user that on acknowledged write data * is synced on disk. */ serverLog(LL_WARNING,"Can‘t recover from AOF write error when the AOF fsync policy is ‘always‘. Exiting..."); exit(1); } else { return; /* We‘ll try again on the next call... */ } else { /* Successful write(2). If AOF was in error state, restore the * OK state and log the event. */ } 。。。。。。 /* Perform the fsync if needed. */ if (server.aof_fsync == AOF_FSYNC_ALWAYS) { // redis_fsync是一个宏，Linux实际为fdatasync，其它为fsync // 所以最好不要将redis.conf中的appendfsync设置为always，这极影响性能 redis_fsync(server.aof_fd); /* Let‘s try to get this data on the disk */ } else if ((server.aof_fsync == AOF_FSYNC_EVERYSEC && server.unixtime > server.aof_last_fsync)) { // 如果已在sync状态，则不再重复 // BIO线程会间隔设置sync_in_progress // if (server.aof_fsync == AOF_FSYNC_EVERYSEC) // sync_in_progress = bioPendingJobsOfType(BIO_AOF_FSYNC) != 0; if (!sync_in_progress) // eversec性能并不那么糟糕，因为它： // 后台方式执行fsync // Redis并不是严格意义上的单线程，实际上它创建一组BIO线程，专门处理阻塞和慢操作 // 这些操作就包括FSYNC，另外还有关闭文件和内存的free两个操作。 // 不像always，EVERYSEC模式并不立即调用fsync， // 而是将这个操作丢给了BIO线程异步执行， // BIO线程在进程启动时被创建，两者间通过bio_jobs和bio_pending两个 // 全局对象交互，其中主线程负责写，BIO线程负责消费。 aof_background_fsync(server.aof_fd); server.aof_last_fsync = server.unixtime; } } |