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

Why create a new tier (aggregate)? Why not expand both existing local 
tiers with a RAID group of 12 (10+2)?

Then, when you add another shelf, you'll just expand the RAID groups 
from 10+2 to 22+2, expanding the local tiers again. (They can be up to 
800TB in size)

Performance-wise with SSDs, the effect of a small RG-size is negligible 
(well, you shouldn't go down to 1+2 or even 3+2. But 10+2, you probably 
don't notice a thing, especially considering, that we're talking about a 
relatively small controller here.

IMHO the management advantages of having fewer aggregates/tiers would be 
the deciding factor here.

There's reasons for more local tiers, e.g. SnapLock, but I guess these 
do not come into play here.


Regarding your mention of "I could add half of them to each existing 
tier/disk group and exceed the best practice by eight SSDs.": No you can't.
26+2 is the maximum to which you can go per RAID group (technically you 
could also do 26+3). So keeping RGs balanced would be the goal here:

aggrx = (22+2) + (10+2)

for both nodes. Then you can expand with another shelf to (22+2) + 
(22+2) per node and will be perfectly balanced regarding RGs and Local 
Tiers...


My 2c


Sebastian


On 27.08.2021 19:34, Rue, Randy wrote:
> 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
>
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