9
25
2014
52

bio,request,request_queue的学习

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

http://www.udpwork.com/item/8437.html

Category: 文件系统 | Tags: | Read Count: 7463
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