Re: [PATCH 2/8] readahead: make default readahead size a kernelparameter

[Jan Kara]

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@xxxxxxx>
> > > 
> > > 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...

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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