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

On Thu, Nov 24, 2011 at 11:28:22PM +0100, Jan Kara wrote:

On Mon 21-11-11 19:35:40, Wu Fengguang wrote:

quoted

On Mon, Nov 21, 2011 at 06:01:37PM +0800, Christoph Hellwig wrote:

quoted

On Mon, Nov 21, 2011 at 05:18:21PM +0800, Wu Fengguang wrote:

quoted

From: Nikanth Karthikesan <redacted>

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.

quoted

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.

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....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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