Re: 2.5.59-mm5

[Nikita Danilov <Nikita@Namesys.COM>]

Andrew Morton writes:

[...]

 > 
 > In this very common scenario, the only way we'll ever get "lumps" of reads is
 > if some other processes come in and happen to want to read nearby sectors. 

Or if you have read-ahead for meta-data, which is quite useful. Isn't
read ahead targeting the same problem as this anticipatory scheduling?

 > In the best case, the size of the lump is proportional to the number of
 > processes which are concurrently trying to read something.  This just doesn't
 > happen enough to be significant or interesting.
 > 
 > But writes are completely different.  There is no dependency between them and
 > at any point in time we know where on-disk a lot of writes will be placed. 
 > We don't know that for reads, which is why we need to twiddle thumbs until the
 > application or filesystem makes up its mind.
 > 

Nikita.
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