[dm-devel] [RFC] [PATCH] add serial keyword to the weightedpath prioritizer

[Hannes Reinecke]

On 07/31/2016 09:26 PM, Christophe Varoqui wrote:
> Ben, Hannes,
>
> Can you review this patch, adding a new 'serial' keyword to the
> weightedpath prioritizer.
>
> I compile-tested it only, as I have no testing environment at hand at
> the moment.
>
> I commited it in a separate 'weightedpath-serial' branch for now.
>
> http://git.opensvc.com/?p=multipath-tools/.git;a=commitdiff;h=4dd16d99281104fc3504ad73626894a5c3702fb3
>
> Thanks,
> Christophe Varoqui
> OpenSVC
>
Well.
In general, sure, fine, I don't have any issues with that.
If the customer wants to diddle with his array that way...

The more general problem I'm seeing is that our current two-layered 
priority setup (path groups with distinct priorities and paths within 
them) might not be leading to issues with larger and more complex scenarios.

ATM we already have the problem that clustered scenarios like this:

Storage node 1(active):
   Path 1 (optimal):
     LUN 1, LUN2
   Path 2 (non-optimal):
     LUN 1, LUN2

Storage node 2(passive):
   Path 1(optimal):
      LUN 1, LUN2
   Path 2(non-optimal):
      LUN 1, LUN2

can not be represented properly with multipath tools.
We are forced to either
a) set 'storage node 2' to 'failed', which would kill
    any cluster instance accessing only 'storage node 2'
or
b) map all priorities from 'storage node 2' to '0',
    thereby losing all priority information

Things become even more convoluted if both storage nodes are in fact 
accessible, or if someone would be using different transports.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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