Hello,
With pools, or any type of striping, it is best practice to keep the same disk type, RAID type, and set size. The reason is because you can have inconsistent performance without. Whether that inconsistent performance is noticeable in your particular environment is not something I can determine but it is possible. The reason is because your worst case performance will always be the lowest common denominator. Say you install 20 x 300GB 15k drives in RAID1 VDISKS. Next, you add 4 x 2.4TB 10K drives in a RAID5. Some, or all (if very small volume), of a given volumes data could live on those 4 x 2.4TB disks and take on the performance characteristics of it. Also, rebalancing is done by the amount of free space available on the drive. So, during a rebalance, a ton of data is going to get moved to those large drives potentially creating some undesired hot spots. I'm giving an extreme example to prove my point. Minor deviations in drive type/sizes or RAID set characteristics can also go completely unnoticed.
Now, onto your real question. I think you have to ask yourself why you chose RAID10 to begin with. If you need the increased write performance that RAID10 can provide (and can tolerate the 50% space reduction) then go with RAID10 on the new disk group, add it to the pool, and call it a day.
If you want as much space as you can get (while having really good data protection) then wipe away what you have in Pool A and create your two RAID6 sets as you mentioned. You don't mention your existing, and new, disk sizes, but if they are the same then I don't think you're going to see a huge issue with the two RAID set sizes. The big thing is to try and stick to the "Power of 2" rule for best performance. This is where the number of data drives is a power of 2, but less than 16 (since MSA can't have more than 16 disks in a VDISK). So, you would do yourself a huge favor by purchasing two more disks and creating two RAID6 sets of 10. You'll have WAY more disk space than RAID10, be meeting the Power of 2 rule, and have very good data protection with double parity drives. I would not mix RAID10 and RAID6 in the same pool. It makes no logical sense and will cause performance inconsistencies. Choose one and go with it. It sounds like you have some flexibility to move data around so maybe you could try RAID6 and, if you didn't like the performance, you could move everything off and recreate on RAID10.
