appendfsync always:主线程调用 write 执行写操作后,后台线程( aof_fsync 线程)立即会调用 fsync 函数同步 AOF 文件(刷盘),fsync 完成后线程返回,这样会严重降低 Redis 的性能(write + fsync)

try_fsync:
 /* Don't fsync if no-appendfsync-on-rewrite is set to yes and there are
 * children doing I/O in the background. */
 if (server.aof_no_fsync_on_rewrite && hasActiveChildProcess())
 return;
 /* Perform the fsync if needed. */
 if (server.aof_fsync == AOF_FSYNC_ALWAYS) {
 /* redis_fsync is defined as fdatasync() for Linux in order to avoid
 * flushing metadata. */
 latencyStartMonitor(latency);
 /* Let's try to get this data on the disk. To guarantee data safe when
 * the AOF fsync policy is 'always', we should exit if failed to fsync
 * AOF (see comment next to the exit(1) after write error above). */
 if (redis_fsync(server.aof_fd) == -1) {
 serverLog(LL_WARNING,"Can't persist AOF for fsync error when the "
 "AOF fsync policy is 'always': %s. Exiting...", strerror(errno));
 exit(1);
 }
 latencyEndMonitor(latency);
 latencyAddSampleIfNeeded("aof-fsync-always",latency);
 server.aof_last_incr_fsync_offset = server.aof_last_incr_size;
 server.aof_last_fsync = server.unixtime;
 atomicSet(server.fsynced_reploff_pending, server.master_repl_offset);
 } else if (server.aof_fsync == AOF_FSYNC_EVERYSEC &&
 server.unixtime > server.aof_last_fsync) {
 if (!sync_in_progress) {
 aof_background_fsync(server.aof_fd);
 server.aof_last_incr_fsync_offset = server.aof_last_incr_size;
 }
 server.aof_last_fsync = server.unixtime;
 }
}