[Lustre-discuss] Read ahead / prefetching

[Andreas Dilger]

On 2010-07-28, at 10:43, Arifa Nisar wrote:
> I am trying to educate myself on prefetching/readahead algorithm for
> Lustre's read. For a starter I only have two simple questions.
> 
> 1 - Does Lustre detect linear or random I/O pattern or it always triggers
> readahead?

The current algorithm detects sequential and strided read patterns.

> 2 - If readahead is triggered, how many pages are read in addition to what
> is necessary? 

It is variable based on how long the same pattern was repeating and the number of pages per request.

I would be happy to have you look at the strided readahead detection code more closely, since I suspect it would not correctly detect stride patterns with odd byte size and offset vs. stride patterns with full-page size and offset.

We disussed a while ago to rewrite the readahead code to be layered better on the client, but haven't had time to work on it yet.


Cheers, Andreas
--
Andreas Dilger
Lustre Technical Lead
Oracle Corporation Canada Inc.