reids持久化机制解释

[YangooSen]

AOF 持久化方式有哪些？ 这里提到的

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）

实际上可能是主线程直接调用 fsync，而没有用到后台线程。7.2版本源码中

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;
    }
}

设置是 AOF_FSYNC_ALWAYS 的时候，会立即调用 redis_fsync(server.aof_fd)；而AOF_FSYNC_EVERYSEC 的分支会才调用 aof_background_fsync(server.aof_fd) 在后台异步执行刷盘