Redis源码分析RDB持久化

RDB持久化：

一、引言：

命令	功能
SAVE	创建RDB文件；阻塞服务器进程
BGSAVE	创建RDB文件；派生子进程，所以不阻塞服务器进程

​ RDB文件以二进制形式存储，通过保存服务器中所有非空数据库以及其中的所有键值对（数据库状态）实现持久化；

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二、RDB文件的创建：

​ BGSAVE和SAVE命令创建RDB文件都是通过rdbSave函数实现的，直接分析这个函数：

/* Save the DB on disk. Return REDIS_ERR on error, REDIS_OK on success */
int rdbSave(char *filename) {
    dictIterator *di = NULL;
    dictEntry *de;
    char tmpfile[256];
    char magic[10];
    int j;
    long long now = mstime();
    FILE *fp;
    rio rdb;
    uint64_t cksum;

    /* 创建文件 */
    snprintf(tmpfile,256,"temp-%d.rdb", (int) getpid());
    fp = fopen(tmpfile,"w");
    if (!fp) {
        redisLog(REDIS_WARNING, "Failed opening .rdb for saving: %s",
            strerror(errno));
        return REDIS_ERR;
    }

    rioInitWithFile(&rdb,fp);
    if (server.rdb_checksum)
        rdb.update_cksum = rioGenericUpdateChecksum;
    /* RDB文件头部几个字节REDIS+version，程序可以在载入时快速判断是否时RDB文件 */
    snprintf(magic,sizeof(magic),"REDIS%04d",REDIS_RDB_VERSION);
    if (rdbWriteRaw(&rdb,magic,9) == -1) goto werr;

    for (j = 0; j < server.dbnum; j++) {
        redisDb *db = server.db+j;
        dict *d = db->dict;
        if (dictSize(d) == 0) continue;
        di = dictGetSafeIterator(d);
        if (!di) {
            fclose(fp);
            return REDIS_ERR;
        }

        /* Write the SELECT DB opcode */
        if (rdbSaveType(&rdb,REDIS_RDB_OPCODE_SELECTDB) == -1) goto werr;
        if (rdbSaveLen(&rdb,j) == -1) goto werr;

        /* Iterate this DB writing every entry */
        while((de = dictNext(di)) != NULL) {
            sds keystr = dictGetKey(de);
            robj key, *o = dictGetVal(de);
            long long expire;
            
            initStaticStringObject(key,keystr);
            expire = getExpire(db,&key);
            if (rdbSaveKeyValuePair(&rdb,&key,o,expire,now) == -1) goto werr;
        }
        dictReleaseIterator(di);
    }
    di = NULL; /* So that we don't release it again on error. */

    /* EOF opcode */
    if (rdbSaveType(&rdb,REDIS_RDB_OPCODE_EOF) == -1) goto werr;

    /* CRC64 checksum. It will be zero if checksum computation is disabled, the
     * loading code skips the check in this case. */
    cksum = rdb.cksum;
    memrev64ifbe(&cksum);
    rioWrite(&rdb,&cksum,8);

    /* Make sure data will not remain on the OS's output buffers */
    fflush(fp);
    fsync(fileno(fp));
    fclose(fp);

    /* Use RENAME to make sure the DB file is changed atomically only
     * if the generate DB file is ok. */
    if (rename(tmpfile,filename) == -1) {
        redisLog(REDIS_WARNING,"Error moving temp DB file on the final destination: %s", strerror(errno));
        unlink(tmpfile);
        return REDIS_ERR;
    }
    redisLog(REDIS_NOTICE,"DB saved on disk");
    server.dirty = 0;
    server.lastsave = time(NULL);
    server.lastbgsave_status = REDIS_OK;
    return REDIS_OK;

werr:
    fclose(fp);
    unlink(tmpfile);
    redisLog(REDIS_WARNING,"Write error saving DB on disk: %s", strerror(errno));
    if (di) dictReleaseIterator(di);
    return REDIS_ERR;
}

• L67~L69：参考[1]；

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三、RDB文件载入：

​ 类似RDB文件的创建，不赘述；

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参考：

1. 函数sync、fsync与fdatasync的总结整理(必看篇)_Linux_脚本之家 (jb51.net)