It has to do with the controller. Each RAID set is 'owned' by a controller with the other as it's backup. Each Controller that owns a RAID set also needs to own a Hot Spare. One hot spare for the controller can back up multiple RAID sets.

So with 18 drives all of them can go into one RAID set so one hot spare needed. Note that if you subsequently add (say) 10 drives more you'll get another RAID set and another hot spare at that time.

I have not seen this documented in this exact way but I have seen enough RAID sets on SAS to see this pattern consistently.

- Larry "DrFranken" Bolhuis

On 4/28/2014 9:47 AM, rob@xxxxxxxxx wrote:
I've been told this:
When using (19) or more disk, (2) drives are set aside for hot spare, this
is an IBm default.

When queried about a link to that info it wasn't forthcoming.

Rob Berendt

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