Message-ID: <40E279F4.4090708@hist.no>
Date: Wed, 30 Jun 2004 10:29:40 +0200
From: Helge Hafting <helge.hafting@hist.no>
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To: Timothy Miller <miller@techsource.com>
CC: linux-kernel@vger.kernel.org
Subject: Re: Block Device Caching
References: <20040630002014.4970b82d@kingfisher.intern.logi-track.com> <40E1FDEC.6020606@techsource.com>
In-Reply-To: <40E1FDEC.6020606@techsource.com>
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Timothy Miller wrote:

>
>
> Markus Schaber wrote:
>
>> This lead us to the conclusion that block devices do not cache, but the
>> filesystem does. But subsequently, I ran some tests on my developer
>> machine (Pentium 4 Mobile Laptop).
>
>
>
> I had kernel experts repeatedly insist to me that block devices were 
> cached, while all of my tests (using dd to or from, say, /dev/sda1 or 
> whatever) indicated that there was absolutely no caching whatsoever.

Well, any cache is dropped when the device is closed. "dd" closes the device
when it finishes.

Try a program that reads the same two blocks (spaced videly apart)
over and over from the same open file descriptor.  With _no_ caching 
you'll see
the drive seeking all the time.  With caching, you won't.

Helge Hafting
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