[Lustre-discuss] Read ahead / prefetching

[Arifa Nisar]

Thanks for the response. If an access pattern triggers page aligned strided
access then how many pages will be readahead? Are there some environment
variables which may give insight about readahead behavior? 

Arifa.

-----Original Message-----
From: Andreas Dilger [mailto:andreas.dilger at oracle.com] 
Sent: Wednesday, July 28, 2010 5:43 PM
To: Arifa Nisar
Cc: lustre-discuss at lists.lustre.org
Subject: Re: [Lustre-discuss] Read ahead / prefetching

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.