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Date: Sun, 04 Nov 2001 23:14:27 -0800
From: Andrew Morton <akpm@zip.com.au>
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To: Jens Axboe <axboe@suse.de>
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Subject: Re: [Ext2-devel] disk throughput
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Jens Axboe wrote:
> 
> On Sun, Nov 04 2001, Andrew Morton wrote:
> > The meaning of the parameter to elvtune is a complete mystery, and the
> > code is uncommented crud (tautology).  So I just used -r20000 -w20000.
> 
> It's the number of sectors that are allowed to pass a request on the
> queue, because of merges or inserts before that particular request. So
> you want lower than that probably, and you want READ latency to be
> smaller than WRITE latency too. The default I set is 8192/16384 iirc, so
> go lower than this -- -r512 -w1024 or even lower just to check the
> results.

Right, thanks.  With the ialloc.c one-liner I didn't touch
elvtune.  Defaults seem fine.

It should the number of requests which are allowed to pass a
request, not the number of sectors!

Well, you know what I mean:   Make it 

	1 + nr_sectors_in_request / 1000

> > This was based on observing the request queue dynamics.  We frequently
> > fail to merge requests which really should be merged regardless of
> > latency.  Bumping the elvtune settings fixed it all.  But once the
> > fs starts writing data out contiguously it's all academic.
> 
> Interesting, the 2.5 design prevents this since it doesn't account
> merges as a penalty (like a seek). I can do something like that for 2.4
> too, I'll do a patch for you to test.

OK.

-