在通用块层中，bio用来描述单一的I/O请求，它记录了一次I/O操作所必需的相关信息，如用于I/O操作的数据缓存位置，I/O操作的块设备起始扇区，是读操作还是写操作等等。struct bio的定义如下

struct bio {
	sector_t		bi_sector;	/* device address in 512 byte
						   sectors */
	struct bio		*bi_next;	/* request queue link */
	struct block_device	*bi_bdev;
	unsigned long		bi_flags;	/* status, command, etc */
	unsigned long		bi_rw;		/* bottom bits READ/WRITE,
						 * top bits priority
						 */

	unsigned short		bi_vcnt;	/* how many bio_vec's */
	unsigned short		bi_idx;		/* current index into bvl_vec */

	/* Number of segments in this BIO after
	 * physical address coalescing is performed.
	 */
	unsigned int		bi_phys_segments;

	unsigned int		bi_size;	/* residual I/O count */

	/*
	 * To keep track of the max segment size, we account for the
	 * sizes of the first and last mergeable segments in this bio.
	 */
	unsigned int		bi_seg_front_size;
	unsigned int		bi_seg_back_size;

	unsigned int		bi_max_vecs;	/* max bvl_vecs we can hold */

	unsigned int		bi_comp_cpu;	/* completion CPU */

	atomic_t		bi_cnt;		/* pin count */

	struct bio_vec		*bi_io_vec;	/* the actual vec list */

	bio_end_io_t		*bi_end_io;

	void			*bi_private;
#if defined(CONFIG_BLK_DEV_INTEGRITY)
	struct bio_integrity_payload *bi_integrity;  /* data integrity */
#endif

	bio_destructor_t	*bi_destructor;	/* destructor */

	/*
	 * We can inline a number of vecs at the end of the bio, to avoid
	 * double allocations for a small number of bio_vecs. This member
	 * MUST obviously be kept at the very end of the bio.
	 */
	struct bio_vec		bi_inline_vecs[0];
};

bi_sector:该I/O操作的起始扇区号

bi_rw:指明了读写方向

bi_vcnt:该I/O操作中涉及到了多少个缓存向量，每个缓存向量由[page,offset,len]来描述

bi_idx:指示当前的缓存向量

bi_io_vec：缓存向量数组

缓存向量的定义：

struct bio_vec {
	struct page	*bv_page;
	unsigned int	bv_len;
	unsigned int	bv_offset;
};

struct request用于描述提交给块设备的I/O请求，bio会动态地添加进request，因此一个request往往会包含若干相邻的bio。

struct request {
	struct list_head queuelist;
	struct call_single_data csd;
	int cpu;

	struct request_queue *q;

	unsigned int cmd_flags;
	enum rq_cmd_type_bits cmd_type;
	unsigned long atomic_flags;

	/* the following two fields are internal, NEVER access directly */
	sector_t __sector;		/* sector cursor */
	unsigned int __data_len;	/* total data len */

	struct bio *bio;
	struct bio *biotail;

	struct hlist_node hash;	/* merge hash */
	/*
	 * The rb_node is only used inside the io scheduler, requests
	 * are pruned when moved to the dispatch queue. So let the
	 * completion_data share space with the rb_node.
	 */
	union {
		struct rb_node rb_node;	/* sort/lookup */
		void *completion_data;
	};

	/*
	 * two pointers are available for the IO schedulers, if they need
	 * more they have to dynamically allocate it.
	 */
	void *elevator_private;
	void *elevator_private2;

	struct gendisk *rq_disk;
	unsigned long start_time;

	/* Number of scatter-gather DMA addr+len pairs after
	 * physical address coalescing is performed.
	 */
	unsigned short nr_phys_segments;

	unsigned short ioprio;

	void *special;		/* opaque pointer available for LLD use */
	char *buffer;		/* kaddr of the current segment if available */

	int tag;
	int errors;

	int ref_count;

	/*
	 * when request is used as a packet command carrier
	 */
	unsigned short cmd_len;
	unsigned char __cmd[BLK_MAX_CDB];
	unsigned char *cmd;

	unsigned int extra_len;	/* length of alignment and padding */
	unsigned int sense_len;
	unsigned int resid_len;	/* residual count */
	void *sense;

	unsigned long deadline;
	struct list_head timeout_list;
	un