Linux块设备驱动（四）————块设备的数据结构与相关操作及I/O调度器

0、数据从内存到磁盘的过程
内存是一个线性的结构，Linux系统将内存分为页。一页最大可以是64KB，但是目前主流的系统页的大小都是4KB。每一页的数据会被先封装成一个段，用bio_vec表示。多个页会被封装成多个段，这些段被组成以一个bio_vec为元素的数组，这个数组用bio_io_vec表示。
bio_io_vec是bio中的一个指针。一个或者多个bio会组成一个request请求描述符。request将被连接到请求队列request_queue中，或者被合并到已经有的请求队列request_queue已有的request中。合并的条件是两个相邻的request请求所表示的扇区位置相邻。最后这个请求队列被处理，将数据写入磁盘。

1、块I/O请求(bio)
数据从内存到磁盘或者从磁盘到内存的过程，叫做I/O操作。内核使用一个核心数据结构来描述I/O操作。bio结构包含了一个段的数据(bio_io_vec)，这个段的数据就是要操作的数据。

/* * main unit of I/O for the block layer and lower layers (ie drivers and * stacking drivers) */struct bio {    /*要传送的第一个扇区*/    sector_t        bi_sector;  /* device address in 512 byte                           sectors */    /*下一个扇区*/    struct bio      *bi_next;   /* request queue link */    struct block_device *bi_bdev;   /*bio对应的块设备*/    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 short      bi_phys_segments;    /* Number of segments after physical and DMA remapping     * hardware coalescing is performed.     */    unsigned short      bi_hw_segments;    unsigned int        bi_size;    /* residual I/O count */    /*     * To keep track of the max hw size, we account for the     * sizes of the first and last virtually mergeable segments     * in this bio     */    unsigned int        bi_hw_front_size;    unsigned int        bi_hw_back_size;    unsigned int        bi_max_vecs;    /* max bvl_vecs we can hold */    struct bio_vec      *bi_io_vec; /* the actual vec list */    bio_end_io_t        *bi_end_io;    atomic_t        bi_cnt;     /* pin count */    void            *bi_private;    bio_destructor_t    *bi_destructor; /* destructor */};

与bio相关的宏

/*于获取目前的页指针*/ bio_page(bio)  /*用于获取目前的页的偏移*/bio_offset(bio) bio_cur_sectors(bio)    

2、请求结构(request)
几个连续的页面会组成一个bio结构，几个相邻的bio结构就会组成一个请求结构(request)。这样就不需要大幅度移动磁头了，节省了I/O操作的时间。

/* * try to put the fields that are referenced together in the same cacheline */ /*请求结构request*/struct request {    struct list_head queuelist; /*请求队列request_queue链表*/    struct list_head donelist;    request_queue_t *q;    unsigned int cmd_flags;    enum rq_cmd_type_bits cmd_type;    /* Maintain bio traversal state for part by part I/O submission.     * hard_* are block layer internals, no driver should touch them!     */    /*要传送的第一个扇区号*/    sector_t sector;        /* next sector to submit */    /*要传送的下一个扇区*/    sector_t hard_sector;       /* next sector to complete */    unsigned long nr_sectors;   /* no. of sectors left to submit */    unsigned long hard_nr_sectors;  /* no. of sectors left to complete */    /* no. of sectors left to submit in the current segment */    unsigned int current_nr_sectors;    /* no. of sectors left to complete in the current segment */    unsigned int hard_cur_sectors;    struct bio *bio; /*指向第一个未完成的bio结构域*/    struct bio *biotail;/*请求链表中最后一个bio*/    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;  /*指向I/O调度器的私有数据1*/    void *elevator_private2;/*指向I/O调度器的私有数据2*/    struct gendisk *rq_disk;  /*指向请求所指向的磁盘*/    unsigned long start_time;    /* Number of scatter-gather DMA addr+len pairs after     * physical address coalescing is performed.     */    unsigned short nr_phys_segments;/*请求的物理段数*/    /* Number of scatter-gather addr+len pairs after     * physical and DMA remapping hardware coalescing is performed.     * This is the number of scatter-gather entries the driver     * will actually have to deal with after DMA mapping is done.     */    unsigned short nr_hw_segments;    unsigned short ioprio;    void *special;    char *buffer;    int tag;    int errors;    int ref_count;    /*     * when request is used as a packet command carrier     */    unsigned int cmd_len;    unsigned char cmd[BLK_MAX_CDB];    unsigned int data_len;    unsigned int sense_len;    void *data;    void *sense;    unsigned int timeout;    int retries;    /*     * completion callback.     */    rq_end_io_fn *end_io;    void *end_io_data;};

3、请求队列(request_queue)
请求队列主要是用来连接对同一块设备的多个request请求结构。还包含块设备所支持的请求类型信息、请求的个数、段的大小、硬件扇区数等与设备相关的信息。

/*内核将请求队列request_queue设计为一个双向链表，链接request 请求*/struct request_queue{    /*     * Together with queue_head for cacheline sharing     */    struct list_head    queue_head;/*连接到request结构，                                表示待处理的请求*/    struct request      *last_merge;    elevator_t      *elevator;  /*电梯调度算法的指针*/    /*     * the queue request freelist, one for reads and one for writes     */    struct request_list rq;/*为分配请求描述符使用的数据结构*/    /*实现驱动程序处理请求的函数*/    request_fn_proc     *request_fn;    /*将一个新的request请求插入请求队列中的方法*/    make_request_fn     *make_request_fn;    prep_rq_fn      *prep_rq_fn;    unplug_fn       *unplug_fn;    merge_bvec_fn       *merge_bvec_fn;    issue_flush_fn      *issue_flush_fn;    prepare_flush_fn    *prepare_flush_fn;    softirq_done_fn     *softirq_done_fn;    /*     * Dispatch queue sorting     */    sector_t        end_sector;    struct request      *boundary_rq;    /*     * Auto-unplugging state     */    struct timer_list   unplug_timer;    int         unplug_thresh;  /* After this many requests */    unsigned long       unplug_delay;   /* After this many jiffies */    struct work_struct  unplug_work;    struct backing_dev_info backing_dev_info;    /*     * The queue owner gets to use this for whatever they like.     * ll_rw_blk doesn't touch it.     */    void            *queuedata;/*指向块设备驱动程序的私有数据的指针*/    /*     * queue needs bounce pages for pages above this limit     */    unsigned long       bounce_pfn;    gfp_t           bounce_gfp;    /*     * various queue flags, see QUEUE_* below     */    unsigned long       queue_flags;    /*     * protects queue structures from reentrancy. ->__queue_lock should     * _never_ be used directly, it is queue private. always use     * ->queue_lock.     */    spinlock_t      __queue_lock;    spinlock_t      *queue_lock;    /*     * queue kobject     */    struct kobject kobj;    /*     * queue settings     */    unsigned long       nr_requests;    /* Max # of requests */    unsigned int        nr_congestion_on;    unsigned int        nr_congestion_off;    unsigned int        nr_batching;    unsigned int        max_sectors;    unsigned int        max_hw_sectors;    unsigned short      max_phys_segments;    unsigned short      max_hw_segments;    unsigned short      hardsect_size;    unsigned int        max_segment_size;    unsigned long       seg_boundary_mask;    unsigned int        dma_alignment;    struct blk_queue_tag    *queue_tags;    unsigned int        nr_sorted;    unsigned int        in_flight;    /*     * sg stuff     */    unsigned int        sg_timeout;    unsigned int        sg_reserved_size;    int         node;#ifdef CONFIG_BLK_DEV_IO_TRACE    struct blk_trace    *blk_trace;#endif    /*     * reserved for flush operations     */    unsigned int        ordered, next_ordered, ordseq;    int         orderr, ordcolor;    struct request      pre_flush_rq, bar_rq, post_flush_rq;    struct request      *orig_bar_rq;    unsigned int        bi_size;    struct mutex        sysfs_lock;};

4、总结
请求队列(request_queue)、请求结构(request)、bio等之间的关系

5、四种调度算法（电梯算法）
内核需要一种调度，使物理相邻的请求尽可能先后执行，这样就可以减少寻找扇区的时间，这种调度就叫做I/O调度。

预期算法(Anticipatory)、最后期限算法、CFQ完全公平队列算法、Noop无操作算法。