linux-kernel - Re: [PATCH 2/8] readahead: make default readahead size a kernel parameter

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Date:	Mon, 28 Nov 2011 10:39:22 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Ankit Jain <radical@...il.com>,
	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Nikanth Karthikesan <knikanth@...e.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH 2/8] readahead: make default readahead size a kernel
 parameter

On Fri, Nov 25, 2011 at 08:36:33AM +0800, Dave Chinner wrote:
> On Thu, Nov 24, 2011 at 11:28:22PM +0100, Jan Kara wrote:
> > On Mon 21-11-11 19:35:40, Wu Fengguang wrote:
> > > On Mon, Nov 21, 2011 at 06:01:37PM +0800, Christoph Hellwig wrote:
> > > > On Mon, Nov 21, 2011 at 05:18:21PM +0800, Wu Fengguang wrote:
> > > > > From: Nikanth Karthikesan <knikanth@...e.de>
> > > > > 
> > > > > Add new kernel parameter "readahead=", which allows user to override
> > > > > the static VM_MAX_READAHEAD=128kb.
> > > > 
> > > > Is a boot-time paramter really such a good idea?  I would at least
> > > 
> > > It's most convenient to set at boot time, because the default size
> > > will be used to initialize all the block devices.
> > > 
> > > > make it a sysctl so that it's run-time controllable, including
> > > > beeing able to set it from initscripts.
> > > 
> > > Once boot up, it's more natural to set the size one by one, for
> > > example
> > > 
> > >         blockdev --setra 1024 /dev/sda2
> > > or
> > >         echo 512 > /sys/block/sda/queue/read_ahead_kb
> > > 
> > > And you still have the chance to modify the global default, but the
> > > change will only be inherited by newly created devices thereafter:
> > > 
> > >         echo 512 > /sys/devices/virtual/bdi/default/read_ahead_kb
> > > 
> > > The above command is very suitable for use in initscripts.  However
> > > there are no natural way to do sysctl as there is no such a global
> > > value.
> >   Well, you can always have an udev rule to set read_ahead_kb to whatever
> > you want. In some respect that looks like a nicer solution to me...
> 
> And one that has already been in use for exactly this purpose for
> years. Indeed, it's far more flexible because you can give different
> types of devices different default readahead settings quite easily,
> and it you can set different defaults for just about any tunable
> parameter (e.g. readahead, ctq depth, max IO sizes, etc) in the same
> way.

I'm interested in this usage, too. Would you share some of your rules?

> Hence I don't think we should treat default readahead any
> differently from any other configurable storage parameter - we've
> already got places to change the per-device defaults to something
> sensible at boot/discovery time....

OK, I'll drop this patch.

Thanks,
Fengguang
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