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.
The multipathing policy depends on the Storage you have. When you storage is active-pasive, the recommended multipathing policy is MRU.
Regards / Saludos
Patricio Cerda !http://www.images.wisestamp.com/linkedin.png!
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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.
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.
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.
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.