Re: [PATCH 10/11] readahead: dont do start-of-file readahead afterlseek()

[david]

On Tue, 2 Feb 2010, Linus Torvalds wrote:

> Rememebr: read-ahead is about filling the empty IO spaces _between_ reads,
> and turning many smaller reads into one bigger one. If you only have a
> single big read, read-ahead cannot help.
>
> Also, keep in mind that read-ahead is not always a win. It can be a huge
> loss too. Which is why we have _heuristics_. They fundamentally cannot
> catch every case, but what they aim for is to do a good job on average.

as a note from the field, I just had an application that needed to be 
changed because it did excessive read-ahead. it turned a 2 min reporting 
run into a 20 min reporting run because for this report the access was 
really random and the app forced large read-ahead.

David Lang
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