Yes you are correct that you can only add two drives at a time. Why it takes so long is all the stripes have to be read and then rewritten in the new stripe configuration. As an example, let's say you are trying to go from a RAID5 in a 4+1 configuration to a RAID5 in a 6+1 configuration (two drives added). So every stripe in the 4+1 configuration (whether it has data on it or not) must be read and then rewritten in the 6+1 configuration. This just takes a long time to do dynamically such that the data is still available for use. Each time you add two more drives, this operation will need to be performed again. The time it takes to perform this operation is dependent on the size of the disk, larger disk will take longer.
This is one of the reasons NetApp E-Series developed Dynamic Disk Pools. It allows customers to perform these dynamic operations in a much quicker and simpler fashion. You would have been able to add all eight disk at once and it would just redistribute data across those disk to maintain balance.
One thing you might try is to increase the priority of this background process. Note though, this will slow down user access to the data. Check the online help as to how to change priorities of operations.
Regards,
Mitch
