Re: [Lsf-pc] [dm-devel] [LSF/MM TOPIC] a few storage topics

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On 2012-01-26, at 8:27 PM, Wu Fengguang wrote:
> On Fri, Jan 27, 2012 at 09:34:49AM +1100, Dave Chinner wrote:
>> On Thu, Jan 26, 2012 at 11:25:56AM -0500, Vivek Goyal wrote:
>>> On Wed, Jan 25, 2012 at 02:35:52PM +0800, Wu Fengguang wrote:
>>>>> It would also be
>>>>> possible to trigger on the size of the device so that the 32MB USB stick
>>>>> doesn't sit busy for a minute with readahead that is useless.
>>>> 
>>>> Yeah, I do have a patch for shrinking readahead size based on device size.
>>> 
>>> Should it be a udev rule to change read_ahead_kb on device based on device
>>> size, instead of a kernel patch?
>> 
>> That's effectively what vendors like SGI have been doing since udev
>> was first introduced, though more often the rules are based on device
>> type rather than size. e.g. a 64GB device might be a USB flash drive
>> now, but a 40GB device might be a really fast SSD....
> 
> Fair enough. I'll drop this kernel policy patch 
> 
>        block: limit default readahead size for small devices
>        https://lkml.org/lkml/2011/12/19/89

Fengguang,
Doesn't the kernel derive at least some idea of the speed of a device
due to the writeback changes that you made?  It would be very useful
if we could derive at least some rough metric for the device performance
in the kernel and use that as input to the readahead window size as well.

Cheers, Andreas





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