Re: to understand the logic of raid0_make_request

[Bill Davidsen <davidsen@tmr.com>]

Neil Brown wrote:

>On Tuesday June 13, liudows2@gmail.com wrote:
>  
>
>>hello,everyone.
>>I am studying the code of raid0.But I find that the logic of
>>raid0_make_request is a little difficult to understand.
>>Who can tell me what the function of raid0_make_request will do eventually?
>>    
>>
>
>One of two possibilities.
>
>Most often it will update bio->bi_dev and bio->bi_sector to refer to
>the correct location on the correct underlying devices, and then 
>will return '1'.
>The fact that it returns '1' is noticed by generic_make_request in
>block/ll_rw_block.c and generic_make_request will loop around and
>retry the request on the new device at the new offset.
>
>However in the unusual case that the request cross a chunk boundary
>and so needs to be sent to two different devices, raidi_make_request
>will split the bio into to (using bio_split) will submit each of the
>two bios directly down to the appropriate devices - and will then
>return '0', so that generic make request doesn't loop around.
>
>I hope that helps.
>
Helps me, anyway, thanks! Wish the comments on stuff like that in 
general were clear, you can see what the code *does*, but you have to 
hope that it's what the coder *intended*. And if you're looking for a 
bug it may not be, so this is not an idle complaint.

Some of the kernel coders think "if it was hard to write it should be 
hard to understand."

-- 
bill davidsen <davidsen@tmr.com>
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979