void loadDataFromDisk(void) {
    long long start = ustime();
    if (server.aof_state == AOF_ON) {
        int ret = loadAppendOnlyFiles(server.aof_manifest);
        if (ret == AOF_FAILED || ret == AOF_OPEN_ERR)
            exit(1);
        if (ret != AOF_NOT_EXIST)
            serverLog(LL_NOTICE, "DB loaded from append only file: %.3f seconds", (float)(ustime()-start)/1000000);
        updateReplOffsetAndResetEndOffset();
    } else {
       // load rdb 

void saveCommand(client *c) {
    if (server.child_type == CHILD_TYPE_RDB) {
        addReplyError(c,"Background save already in progress");
        return;
    }

    server.stat_rdb_saves++;

    rdbSaveInfo rsi, *rsiptr;
    rsiptr = rdbPopulateSaveInfo(&rsi);
    if (rdbSave(SLAVE_REQ_NONE,server.rdb_filename,rsiptr,RDBFLAGS_NONE) == C_OK) {
        addReply(c,shared.ok);
    } else {
        addReplyErrorObject(c,shared.err);
    }
}

    // bgsaveCommand
    if (server.child_type == CHILD_TYPE_RDB) {
        addReplyError(c,"Background save already in progress");
    } else if (hasActiveChildProcess() || server.in_exec) {
        if (schedule || server.in_exec) {
            server.rdb_bgsave_scheduled = 1;
            addReplyStatus(c,"Background saving scheduled");
        } else {
            addReplyError(c,
            "Another child process is active (AOF?): can't BGSAVE right now. "
            "Use BGSAVE SCHEDULE in order to schedule a BGSAVE whenever "
            "possible.");
        }
    } else if (rdbSaveBackground(SLAVE_REQ_NONE,server.rdb_filename,rsiptr,RDBFLAGS_NONE) == C_OK) {
        addReplyStatus(c,"Background saving started");
    } else {

# 这个保存点配置表示每60秒，如果数据发生了1000次以上的变动，Redis就会自动保存快照文件
save 60 1000
# 保存点可以设置多个，Redis的配置文件就默认设置了3个保存点
# 格式为：save <seconds> <changes>
# 可以设置多个。
save 900 1 #900秒后至少1个key有变动
save 300 10 #300秒后至少10个key有变动
save 60 10000 #60秒后至少10000个key有变动

/* Write the given command to the aof file.
 * dictid - dictionary id the command should be applied to,
 *          this is used in order to decide if a `select` command
 *          should also be written to the aof. Value of -1 means
 *          to avoid writing `select` command in any case.
 * argv   - The command to write to the aof.
 * argc   - Number of values in argv
 */
void feedAppendOnlyFile(int dictid, robj **argv, int argc) {
    sds buf = sdsempty();

    serverAssert(dictid == -1 || (dictid >= 0 && dictid < server.dbnum));

    /* Feed timestamp if needed */
    if (server.aof_timestamp_enabled) {
        sds ts = genAofTimestampAnnotationIfNeeded(0);
        if (ts != NULL) {
            buf = sdscatsds(buf, ts);
            sdsfree(ts);
        }
    }

    /* The DB this command was targeting is not the same as the last command
     * we appended. To issue a SELECT command is needed. */
    if (dictid != -1 && dictid != server.aof_selected_db) {
        char seldb[64];

        snprintf(seldb,sizeof(seldb),"%d",dictid);
        buf = sdscatprintf(buf,"*2\r\n$6\r\nSELECT\r\n$%lu\r\n%s\r\n",
            (unsigned long)strlen(seldb),seldb);
        server.aof_selected_db = dictid;
    }

    /* All commands should be propagated the same way in AOF as in replication.
     * No need for AOF-specific translation. */
    buf = catAppendOnlyGenericCommand(buf,argc,argv);

    /* Append to the AOF buffer. This will be flushed on disk just before
     * of re-entering the event loop, so before the client will get a
     * positive reply about the operation performed. */
    if (server.aof_state == AOF_ON ||
        (server.aof_state == AOF_WAIT_REWRITE && server.child_type == CHILD_TYPE_AOF))
    {
        server.aof_buf = sdscatlen(server.aof_buf, buf, sdslen(buf));
    }

    sdsfree(buf);
}

// flushAppendOnlyFile
latencyStartMonitor(latency);
nwritten = aofWrite(server.aof_fd,server.aof_buf,sdslen(server.aof_buf));
latencyEndMonitor(latency);

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.mstime;
        atomicSet(server.fsynced_reploff_pending, server.master_repl_offset);
    } else if (server.aof_fsync == AOF_FSYNC_EVERYSEC &&
               server.mstime - server.aof_last_fsync >= 1000) {
        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.mstime;
    }

/* ----------------------------------------------------------------------------
 * AOF background rewrite
 * ------------------------------------------------------------------------- */

/* This is how rewriting of the append only file in background works:
 *
 * 1) The user calls BGREWRITEAOF
 * 2) Redis calls this function, that forks():
 *    2a) the child rewrite the append only file in a temp file.
 *    2b) the parent open a new INCR AOF file to continue writing.
 * 3) When the child finished '2a' exists.
 * 4) The parent will trap the exit code, if it's OK, it will:
 *    4a) get a new BASE file name and mark the previous (if we have) as the HISTORY type
 *    4b) rename(2) the temp file in new BASE file name
 *    4c) mark the rewritten INCR AOFs as history type
 *    4d) persist AOF manifest file
 *    4e) Delete the history files use bio
 */
int rewriteAppendOnlyFileBackground(void) {
    pid_t childpid;

    if (hasActiveChildProcess()) return C_ERR;

    if (dirCreateIfMissing(server.aof_dirname) == -1) {
        serverLog(LL_WARNING, "Can't open or create append-only dir %s: %s",
            server.aof_dirname, strerror(errno));
        server.aof_lastbgrewrite_status = C_ERR;
        return C_ERR;
    }

    /* We set aof_selected_db to -1 in order to force the next call to the
     * feedAppendOnlyFile() to issue a SELECT command. */
    server.aof_selected_db = -1;
    flushAppendOnlyFile(1);
    if (openNewIncrAofForAppend() != C_OK) {
        server.aof_lastbgrewrite_status = C_ERR;
        return C_ERR;
    }

    if (server.aof_state == AOF_WAIT_REWRITE) {
        /* Wait for all bio jobs related to AOF to drain. This prevents a race
         * between updates to `fsynced_reploff_pending` of the worker thread, belonging
         * to the previous AOF, and the new one. This concern is specific for a full
         * sync scenario where we don't wanna risk the ACKed replication offset
         * jumping backwards or forward when switching to a different master. */
        bioDrainWorker(BIO_AOF_FSYNC);

        /* Set the initial repl_offset, which will be applied to fsynced_reploff
         * when AOFRW finishes (after possibly being updated by a bio thread) */
        atomicSet(server.fsynced_reploff_pending, server.master_repl_offset);
        server.fsynced_reploff = 0;
    }

    server.stat_aof_rewrites++;

    if ((childpid = redisFork(CHILD_TYPE_AOF)) == 0) {
        char tmpfile[256];

        /* Child */
        redisSetProcTitle("redis-aof-rewrite");
        redisSetCpuAffinity(server.aof_rewrite_cpulist);
        snprintf(tmpfile,256,"temp-rewriteaof-bg-%d.aof", (int) getpid());
        if (rewriteAppendOnlyFile(tmpfile) == C_OK) {
            serverLog(LL_NOTICE,
                "Successfully created the temporary AOF base file %s", tmpfile);
            sendChildCowInfo(CHILD_INFO_TYPE_AOF_COW_SIZE, "AOF rewrite");
            exitFromChild(0, 0);
        } else {
            exitFromChild(1, 0);
        }
    } else {
        /* Parent */
        if (childpid == -1) {
            server.aof_lastbgrewrite_status = C_ERR;
            serverLog(LL_WARNING,
                "Can't rewrite append only file in background: fork: %s",
                strerror(errno));
            return C_ERR;
        }
        serverLog(LL_NOTICE,
            "Background append only file rewriting started by pid %ld",(long) childpid);
        server.aof_rewrite_scheduled = 0;
        server.aof_rewrite_time_start = time(NULL);
        return C_OK;
    }
    return C_OK; /* unreached */
}