原创文章是freas_1990。转载请注明出处：http://blog.csdn.net/freas_1990/article/details/42052813

Redis唯一的那个key的value是swap文件上的时候，该key的value指向的RedisObject将会改变成VMPointer。VMPointer保存了该value在磁盘文件上的信息，包含起始页面的偏移和连续的页面数等。

typedef struct redisObject {

    unsigned type:4;

    unsigned storage:2;     /* REDIS_VM_MEMORY or REDIS_VM_SWAPPING */

    unsigned encoding:4;

    unsigned lru:22;        /* lru time (relative to server.lruclock) */

    int refcount;

    void *ptr;

    /* VM fields are only allocated if VM is active, otherwise the

     * object allocation function will just allocate

     * sizeof(redisObjct) minus sizeof(redisObjectVM), so using

     * Redis without VM active will not have any overhead. */

} robj;

typedef struct vmPointer {

    unsigned type:4;

    unsigned storage:2; /* REDIS_VM_SWAPPED or REDIS_VM_LOADING */

    unsigned notused:26;

    unsigned int vtype; /* type of the object stored in the swap file */

    off_t page;         /* the page at witch the object is stored on disk */

    off_t usedpages;    /* number of pages used on disk */

} vmpointer;

将该key的value导入内存的逻辑例如以下：

robj *vmReadObjectFromSwap(off_t page, int type) {

    robj *o;

    if (server.vm_enabled) pthread_mutex_lock(&server.io_swapfile_mutex);

    if (fseeko(server.vm_fp,page*server.vm_page_size,SEEK_SET) == -1) {

        redisLog(REDIS_WARNING,

            "Unrecoverable VM problem in vmReadObjectFromSwap(): can't seek: %s",

            strerror(errno));

        _exit(1);

    }

    o = rdbLoadObject(type,server.vm_fp);

    if (o == NULL) {

        redisLog(REDIS_WARNING, "Unrecoverable VM problem in vmReadObjectFromSwap(): can't load object from swap file: %s", strerror(errno));

        _exit(1);

    }

    if (server.vm_enabled) pthread_mutex_unlock(&server.io_swapfile_mutex);

    return o;

}

到这里，redis VM已经指出的精髓，明白了吧？