[dm-devel] Powerpath vs dm-multipath - two points of FUD?

[Hannes Reinecke]

Hi Jerome,

On 09/14/2014 08:44 PM, Levy, Jerome wrote:
>> Firstly, apologies if this is a common topic and my intentions are not 
>> to start a flame war. I've googled extensively but haven't found
>> specific information to address my queries, so I thought I would turn here.
> 
> At the risk of getting involved in a religious discussion (disclaimer: I am an EMC employee
> and was an advanced support engineer for both PowerPath and
dm-multipath) I thought I'd pass
> along a few thoughts that might help:
> 
> PowerPath costs money. dm-multipath is included with the OS distro.
> 
[ .. ]
> 
> PowerPath contains proprietary load sensing and balancing algorithms which may help performance
> in a given situation. (It does. I've seen them.) YMMV.
> 
Unfortunately I'll have to agree with Jerome here.
If you have in-depth knowledge of the array you can cut some corners
and optimize for certain scenarios.
We haven't, and so we can't.

Neither can we estimate how _likely_ such a scenario is;
We live on the assumption that it's not, and that most customers are
happily living with the standard setups.
Surprisingly enough, most do :-)

> Request size and other switching options  can be useful in a number
> of specific situations -- some may call them corner cases, but
streaming
> media, heavy database backups, large dataset transfers, and others
have
> been shown to benefit from alternate PowerPath policies.
> 
See above. Of course.

> As Hannes points out, PowerPath is supported by EMC. If things don't work,
> you have someone to call. That can be comforting in the middle of
the night :)
> 
But so is device-mapper multipath :-)
Sorry.

> I've seen both PowerPath and native multipath solutions provide a lot of
> value in different ways,  and the decision as to which to use is
not always
> clear-cut. Hope this helps!
> 
Thanks for that. It surely helps.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare at suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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