本文原创为freas_1990，转载请表明出处：http://blog.csdn.net/freas_1990/article/details/42052813

Redis的某一个key的value被swap到文件上的时候，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，包括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;

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;
}

点破Redis的VM