点破Redis的VM

我就是我 2022-08-10 01:00 133阅读 0赞

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;

将该key的value导入内存的逻辑如下：

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的精髓已经点破了，懂了吧？