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

[Christophe Varoqui]

On Mon, Aug 1, 2016 at 9:49 AM, Hannes Reinecke <hare at suse.de> wrote:

> 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.
>
> Would something like "prio alua+weightedpath" produce correct priorities
for the path grouping ? where priorities reported by alua would be added to
those reported by weighted path. That syntax extension would reduce the
need to develop more complex prioritizers.

The prio_args to prio mapping, wouldn't be nice though.

Concerning transports, we can also extend the weightedpath prioritizer, and
if the multi-prioritizer setup is implemented we could even weight on
serial+transport with a "prio weighedpath+weightedpath" setup.
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