点破Redis的VM

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;
}