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-psm... 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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