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Questions
I have a number of customers asking me these types question:

Not sure about best practice but there is common sense - will that do? Restrictions are included with the VIOS announcement "Read Me" notes, which you can find here for the current VIOS 2.2.1.3

• http://www.ibm.com/support/docview.wss?uid=hpc1vios117f5701
• Look for the VIOS Shared Storage Pool requirements section.
• Is 512 TB total pool size and 1024 LUNs enough for now?!!!
  

The SSP will spread out the LU disk space across all the LUNs in the pool

• Off the record and assuming pool LUNs are the same size then it is a round-robin type algorithm at the 64 MB size block.
• This will get you thinking the right way but the developers reserve the right to change this in the future and without telling us.
  

If in the underlying SAN disk subsystems, you are already doing spreading the LUNs around a lot of disks (so you have lots of spindles) then the number of LUNs is not critical. If a single large LUN would map to just a handful of disks then use more LUNs to get more disks involved with I/O for the pool. That would be common sense.  Of course, disk sub-systems with large read and write caches help with I/O rates as well as lots of spindles.

Having said that I would go for 5 x 1 TB or 10 x 512 GB - then there is a natural size to increase the pool later on (1 GB or 512 MB).  You might have a preferred default minimum LUN size in your computer room - like we use 128 GB but that is a little small for 5 TB but 20 at 256 GB is OK too.  Common sense says not just one 5 TB LUN and not loads (like a 100s) as that increases setup work and harder monitoring.

 

I hope that helps, cheers Nigel