点破Redis的VM

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？