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

>>>>> "Randy" == Randy Rue <[email protected]> writes:

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

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

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

Don't you mean 'Raid Group' here?  In any case, I'd just split the
disks in half and add them to the aggregates on each head, to create
some new RGs.

But it would help if you could show your current setup with:

   storage aggregate show-status

I've got an a200 running 9.3.P10 (yes, I know it's old... sue me.) and
we haven't had any issues, but I admit we haven't grown it either.  

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

I don't think you'll notice any performance problems, the issue really
comes down to overhead you lose due to having to dedicate more disks
to parity.  

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

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

Once you add another shelf after this shelf, you'll end up balancing
things since you'll ideally fill your raidgroups to a larger number of
data disks.  

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

I don't think you'll notice any problems, if only because the A220
isn't the highest performing system anyway.  If you've got the time,
setup a new aggregate on one head with the same setup as currently,
then partly fill it and run some performance tests.

Good luck,
John