bio：代表了一个io请求
request：一个request中包含了一个或多个bio，为什么要有request这个结构呢？它存在的目的就是为了进行io的调度。通过request这个辅助结构，我们来给bio进行某种调度方法的排序，从而最大化地提高磁盘访问速度。
request_queue：每个磁盘对应一个request_queue.该队列挂的就是request请求。
具体如下图：(有颜色方框头表示数据结构的名字)
请求到达block层后，通过generic_make_request这个入口函数，在通过调用一系列相关的函数（具体参见我另一篇博客）把bio变成了request。具体的做法如下：如果几个bio要读写的区域是连续的，即积攒成一个request（一个request上挂多个连续的bio，就是我们通常说的“合并bio请求”），如果一个bio跟其他的bio都连不上，那它就自己创建一个新的request，把自己挂在这个request下。当然，合并bio的个数也是有限的，这个可以通过配置文件配置。
对于上段补充一点：上层的一次请求可能跨越了多个扇区，形成不连续的扇区段，那么该请求构造的每个bio对应着一个连续的扇区段。故一个请求可以构造出多个bio。
合并后的request放入每个设备对应的request_queue中。之后设备驱动调用peek_request从request_queue中取出request，进行下一步处理。
这里要注意的是，在实现设备驱动时，厂家可以直接从request_queue中拿出排队好的request，也可以实现自己的bio排队方法，即实现自己的make_request_fn方法，即直接拿文件系统传来的bio来自己进行排队，按需设计，想怎么排就怎么排，像ramdisk,还有很多SSD设备的firmware就是自己排。（这里就和我另一篇博客，说到为什么generic_make_request要设计成一层递归调用联系起来）。
网上有个问答也是我的疑惑：这里我写一遍，加强记忆
bio结构中有bio_vec数组结构，该结构的的数组可以指向不同的page单元，那为什么不在bio这一级就做了bio合并工作，即把多个bio合并成一个bio，何必加入一个request这么麻烦？
答：每个bio有自己的end_bio回调，一旦一个bio结束，就会对自己进行收尾工作，如果合并了，或许有些bio会耽误，灵活性差。
参考博客：
http://blog.csdn.net/flyingcloud_2008/article/details/5818995
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