"RAID group shows differences in size counts"

that looks like you have different size drives.  Take a visual, looks like you have 26 600gb drives (560gb usable) and probably 2x300GB SSD drives. Most likely is that you have a hybrid aggregate.  Typically the SSD drives are connected to the aggregate in a flash pool scenario for really good read cache.

So what should be my next step, do I need to replace those 2 different drive from that aggregate? What is the best practice and why?

You can't have Flash Pool with only 2 SSD drives. They need to be setup with the same parity as the rest of the aggregate which should be RAID-DP. Not to mention you should also have a hot spare since it is a read/write cache. And what is up with the 28 disks? While a SAS/FC RAID Group is up to 28 disks, the disk shelves hold 24 disks. I can't imagine someone buying an extra disk shelf for only 4 disks.

What model filer and disk shelf (if you have a separate disk shelf) do you have? Can you also post a screen shot of aggregate and raid group details?

@Kaps, that's incorrect you can configure RAID4 for your SSD flash pool and RAID-DP for your regular drives, that said you need a min. of 2 drives so you can do 2 drives, 1 parity, and 1 hot spare.

Otherwise the drives are useless but typically that's what they're sold for unless there is another shelf with a few more SSD drives that aren't being used.

And how would you go about setting up a RAID 4 for just the Flash Pool? I have three NetApps using Flash Pool. All aggregates are using RAID-DP, which is recommended. As you can see in the screen shot, the RAID settings apply to both the Data RAID group and Cache RAID group. Changing the RAID Type affects both.
Aggregate.png

OK, you seem to be right about the RAID 4.

https://library.netapp.com/ecmdocs/ECMP1368404/html/GUID-C31EDDB7-7DDD-4042-995C-B93CF0F4B9DA.html

Now I need to have a serious talk with my VAR as to why this was never mentioned. I would like another 200 GB in my Flash Pool. :-)

Thank You all.

The Raid 4 option became available in cDot 8.2 I believe.  To redo the aggregate you would have to blow away and rebuild. I would also upgrade to 8.3 as it's a lot easier to upgrade to newer builds afterwards.

I thought based on that NetApp article you can convert the Flash Pool to RAID4 without blowing away the aggregate?

My fault, I was thinking about 7 mode and having the raid pool on the root aggregate.  I had to blow it away in that instance.

Hmm, well ours is 7-mode but it is aggr1 that has the flash pool.