Redis的appendfsync参数详解
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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; } } |
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