linux-kernel - Re: [PATCH] sysctl: Add a feature to drop caches selectively

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Date:	Fri, 27 Jun 2014 11:04:34 +0200 (CEST)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Matthias Schniedermeyer <ms@...d.de>
cc:	Artem Bityutskiy <dedekind1@...il.com>,
	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>,
	Dave Chinner <david@...morbit.com>,
	Thomas Knauth <thomas.knauth@....de>,
	David Rientjes <rientjes@...gle.com>,
	Maksym Planeta <mcsim.planeta@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sysctl: Add a feature to drop caches selectively

On Fri, 27 Jun 2014, Matthias Schniedermeyer wrote:

> Date: Fri, 27 Jun 2014 10:41:39 +0200
> From: Matthias Schniedermeyer <ms@...d.de>
> To: Luká? Czerner <lczerner@...hat.com>
> Cc: Artem Bityutskiy <dedekind1@...il.com>,
>     Bernd Schubert <bernd.schubert@...m.fraunhofer.de>,
>     Dave Chinner <david@...morbit.com>, Thomas Knauth <thomas.knauth@....de>,
>     David Rientjes <rientjes@...gle.com>,
>     Maksym Planeta <mcsim.planeta@...il.com>,
>     Alexander Viro <viro@...iv.linux.org.uk>, linux-fsdevel@...r.kernel.org,
>     linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] sysctl: Add a feature to drop caches selectively
> 
> On 26.06.2014 13:57, Luká? Czerner wrote:
> 
> > > So if the authors want to sell this new interface (in whatever form) to
> > > the kernel community, they should start with providing a solid use-case,
> > > with some more details, explore alternatives and show how the
> > > alternatives do not work for them.
> > 
> > Yes please, let's see some solid use-case for this.
> 
> Personally i would want it to verify files after copying them:
> Especially while moving files:
> - Copy a file
> - <drop cache>
> - Verify that it really is correct on stable storage
> - Remove original file

I assume you're using cp to copy a file, not your own program. In
that case can we make cp optionally use direct io ? It seems that it
would solve your problem in very elegant way.

-Lukas

> 
> Currently i choose either of the 3 ways:
> - drop_caches
> - umount/mount
> - Write more data than memory in machine (Which is only an 
> approximnation and you have to verify in the same order the files were 
> written, so it is likely that any cache was thrashed in the meantime)
> 
> But having a way to selectivly "destory" the cache of a file would make 
> this task easier.
> 
> 
> 
> 
>