That depends greatly on the values of N and n, as well as
the RAID group size of the aggregate.
Done according to best practices, there will be almost no
difference. Done poorly, it can make all the
difference in the world.
My personal view (I'm in no position to speak officially
for NetApp) on this to add disks to an aggregate in one of 2
multiples:
1) a whole RG at a time
2) half a RG at a time.
This typically allows for a sufficient number of free disks
such that you should not expect any noticable
performance difference. I realize that not all sites
can implement this, but let's look at a worst practice:
N = n+1
Fill up the aggregate, then add 1 disk. Ouch!
This hurts! So you've seen what I consider to be the
best
case for adding space, and you've seen the worst
case. So how close you are to these extremes should
give you can idea of what to expect.
I know this isn't a simple answer, but I believe it to be
accruate.
-- Adam Fox
adamfox@netapp.com
Is there any difference between
creating an aggregate on a certain number of disks (say n) , and then later
expanding the aggregate to N disks, as opposed to creating the initial
aggregate on N disks?
Suresh