Message-ID: <3F5CE3E6.8070201@upb.de>
Date: Mon, 08 Sep 2003 22:17:42 +0200
From: =?ISO-8859-1?Q?Sven_K=F6hler?= <skoehler@upb.de>
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To: Paul Clements <Paul.Clements@SteelEye.com>
CC: Pavel Machek <pavel@suse.cz>, linux-kernel@vger.kernel.org
Subject: Re: [NBD] patch and documentation
References: <3F5CB554.5040507@upb.de> <20030908193838.GA435@elf.ucw.cz> <3F5CE0E5.A5A08A91@SteelEye.com>
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>>The patch also looks harmless enough for applying ;-).
> 
> Harmless enough, although I'm not sure it really makes that much
> difference. The max_sectors being set to 255 doesn't, by itself, explain
> the back and forth 127k, 1k request thing. Typically what you'll see is
> 127k, 127k, 127k, etc. and then some odd sized request at the end. Or
> the device gets unplugged anyway at some point and there are odd sized
> requests scattered throughout...that's especially going to be true if
> the reads or writes are from an actual disk, rather than /dev/null. I
> may be just coincidence that setting max_sectors to 256 actually helps.
> Also, are we sure that all those requests you're seeing are of the same
> type (all reads, all writes)?

Well, i guess the cache uses a value of 256 sectors to do read-ahead and 
such. I used dd if=/dev/nbd/0 of=/dev/null bs=X with both X=1 and X=1M.
Both with the same result. That the 1byte requests join together to 
bigger ones can only be explained with read-aheads strategies.
Anyway, the result is always the same:

without patch: 127KB, 1KB, 127KB, 1KB
with path: 128KB, 128KB, 128KB

As long as dd doesn't write i'm sure that i didn't see any write 
requests. In addition it is a very regular pattern.
If it is really the case that the cache reads 256 sectors and the 
default limit is 255, than this would also happen for all other 
block-devices. In addition it would be a good thing to look up if the 
cache takes the max_sectors stuff into accout while determining the 
amout of sectors it reads ahead.


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