Re: [PATCH v4 0/1] multipath-tools: Prioritizer based on a latency algorithm

[Martin Wilck <mwilck@suse.com>]

Hello Yang,

> > Actually, you're not alone here; several other storage array setups
> > suffer from the same problem.
> > 
> > Eg if you have a site-failover setup with two storage arrays at
> > different locations the problem is more-or-less the same;
> > both arrays potentially will be displaying identical priority
> > information, despite one array being remote.
> > 
> 
> It's up to the value set of the argument "latency_interval".For
> example,
> If latency_interval=10ms, the paths will be grouped in priority
> groups
> with path latency 0-10ms, 10-20ms, 20-30ms, etc. If the argument
> "latency_interval" is set to appropriate value and the distance
> between
> two arrays is not enough far, two priorities may be the same, But
> it's
> OK, because between two arrays, the gap of average path latency is
> very
> small and tolerable.

I wonder if it would make sense to use "logarithmically" scaled latency
intervals here. It wouldn't make a large difference whether the latency
is 1ms or 2ms, but if we have paths where the latencies differ by order
of magnitude, it would be very important to make a distinction. With
the current linear intervals, it would be hard to get this right (us
interval size would result in too many intervals, and sec interval size
wouldn't allow a distinction between us and ms latencies).

By using logarithmic scale, you could setup the latency intevals e.g
like this: 

  < 10us, 10us-100us, 100us-1ms, 1ms-10ms, 10ms-100ms, > 100ms

IMO that would be better, in particular if the latencies differ
strongly between paths.

Regards
Martin


-- 
Dr. Martin Wilck <mwilck@suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)

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