options for adding a shelf/disks to an AFF-A220

Hi All,

I've cabled up the second external shelf for our AFF-A220 and it sees 
the disks. Now I'm at a fork in the road.

We currently have two local data tiers made up of 24 disks each, one on 
each node.

Best practices at 
https://docs.netapp.com/ontap-9/index.jsp?topic=%2Fcom.netapp.doc.dot-cm-psmg%2FGUID-EE30CDA9-095F-4DD1-9A29-BB9F0E55DB4F.html 
say a disk group of SSDs should be from 20-28 SSDs each.

If I go to create a new local tier the default is to create two new 
tiers, one on each node. That would give me two disks groups of 12, 
below the best practice. How badly would performance suffer? Any other 
caveats? Reliability?

I could put them all on one disk group, and on one node? Bleah.

I could add half of them to each existing tier/disk group and exceed the 
best practice by eight SSDs. I believe I've seen discussions (get it?) 
claiming that top limit is flexible with SSDs, was it here? But when I 
add another shelf I'll have the same problem.


Seems like the clearest path forward for consistency and future 
expansions is to add two tiers with small disk groups. So, can anyone 
guess if I'll have a significant performance impact? Any other caveats?


Grateful for any guidance,

Randy in Seattle