claudiofolu
Contributor
Contributor

Fixed or MRU?

Just a question…using IBM DS4x00 SANs, you

have to set a preferred path in IBM Storage Manager, let’s say you

choose SP-A (Storage Processor A) for accessing LUN 1 and SP-B for

accessing LUN 2 and you choose MRU as policy within vCenter

(this is the default setup). When an host is

rebooted, both paths become unavailable and the one which is restored

first become active for VC. It could happen that is SP-B, so that

creates a problem since both LUN (1 & 2) are accessed through SP-B,

MRU doesn’t fail back and the SAN reports an error because LUN 1 isn’t

anymore accessed through preferred path. That’s the situation I faced

some times and I ended setting Fixed Path in VC to preserve preferred

path setting in Storage Manager. How would you do in this case?

Thanks for feedback.

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5 Replies
pcerda
Virtuoso
Virtuoso

Hi,

The multipathing policy depends on the Storage you have. When you storage is active-pasive, the recommended multipathing policy is MRU.




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claudiofolu
Contributor
Contributor

Hello Patricio, thanks for your answer

I know that the multipathing drivers depends on the storage vendor, my case for IBM ds4000 VMware recomend MRU. The situation I explained before is what I´m trying to solve or listen similar experiences.

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peterdabr
Hot Shot
Hot Shot

I'm not familiar with IBM Storage Manager, but if there is a

requirement to set a preferred path on the storage manager, your only

option seems to be Fixed Path.

Btw, if your storage suports ALUA mode you

could 'advertise' both LUNs via both SPs and use either Fixed or MRU policy.

Peter

[

|http://www.logicworks.net/]

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MauroBonder
Leadership
Leadership

There  is an old manual referring to VMware in ESX2.x TotalStorage (Line  Storage IBM) ( http://www.redbooks.ibm.com/redbooks/pdfs/sg246434.pdf)  that talks about best practices for the new versions do not  change much. Actually it is the recommended active / passive - MRU. || active / active - Fixed

To set this up
   1. Click Configuration> Storage.
   2. Right-click on the datastore, and select Properties.
   3. Click Manage Paths and choose the Preferred path.

More informations: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=100941...

*Please, don't forget the awarding points for "helpful" and/or "correct" answers. *Por favor, não esqueça de atribuir os pontos se a resposta foi útil ou resolveu o problema.* Thank you/Obrigado
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ShannonZ
VMware Employee
VMware Employee

Hi,

AFAIK,  if you select FIXED policy for A/P array, after the preferred path recovered, VMkernel will attempt to resume using the preferred path. This can cause path thrashing or failure when another SP now owns the LUN.

Or maybe you can try with FIXED AP policy, aka VMW_PSP_FIXED_AP. With it, there's a path-thrashing avoidance algorithm, VMkernel resumes using the preferred path, but only if the algorithm allows the fail-back.

MRU is default policy for A/P array, and the behavior you described is normal and expected per its functionality.

Regards,

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