Most of the big hardware vendors, HP, Dell, etc. support expanding an existing RAID array but this will not automatically expand your VMFS volume. Once your RAID array is expanded the only way to expand your existing VMFS volume is to add extents. Extents are generally not recommended but because this is local disk and not SAN disk they might be OK to use in your situation. The reason they are not recommended in a SAN environement is because if you lose a LUN that was used to extend your VMFS volume it will take down your whole VMFS volume, when using local disk there is alot less chance of that happening. If you do create a separate RAID group and VMFS volume it does add a little bit more security though. If there is a problem with your 2nd RAID group (in the unlikely event that 2 drives fail) it will not effect your first RAID group and the VM's on it. A controller failure however will take everything down, also a problem with your first RAID group will take everything down since ESX will be installed on the first RAID group. So long answer short I would just expand your existing array and add an extent to expand your VMFS volume to get maximum usage out of your storage.
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Thanks, Eric
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http://vmware-land.com
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