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

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Date:	Mon, 21 Nov 2011 19:35:40 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	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>,
	Dave Chinner <david@...morbit.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 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.

Thanks,
Fengguang
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