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Re: [dm-devel] Re: [PATCH 3/3] dm-mpath: add service-time oriented dynamic load balancer

Hi Alasdair,

On 05/18/2009 08:46 PM +0900, Alasdair G Kergon wrote:
> On Fri, May 15, 2009 at 12:09:21PM +0900, Kiyoshi Ueda wrote:
>> However, if I limit the 'perf' value in smaller range (e.g. 0 to 100),
>> such overflow shouldn't happen.  So I should be able to use
>> multiplication.  Also, I don't need to use size_t for 'perf',
>> because 'unsinged' should be enough for such small values.
> I think a small range would be adequate - look at the number sizes and
> go 0-100 or 0-1000 as appropriate.
> Thinking of naming, is it 'relative_throughput' ?

Yes, 'relative_throughput' may be better naming.  I'll take it.

> However, is it also going to be possible to extend this target to measure the
> the actual throughput?  If we did that, would we use a special value to 
> indicate it or add a new table parameter and keep the 'perf' ones as a
> manual adjustment factor applied on top of the calculated one?

The older version of dm-service-time calculated the actual throughput
dynamically using part_stat:

But the calculated values often had some noises and it didn't work well.
(e.g. Once the throughput value of a path was calculated so small,
      the path was never used, and the small throughput value was never
      updated, and the path was never used, and ...)

So I think doing such calculations in user-space and simplifying
the kernel-code are better.
For dynamically changing path's bandwidth, user-space daemon can
measure the throughput periodically and notify the dm device.
I've been thinking of adding a new dm message handler to update
'relative_thoughput' in future.
What do you think?

Kiyoshi Ueda

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