Hello, for NL-SAS the preferred count for R6 is either 6+2 or 14+2, so purchasing 7 drives is odd.

Ideally you want to have the same count as exists in the tier, to avoid hot spots from uneven private raid groups.

I think the multiple that can be used (non-preferred)  for R6 are 2+2, 4+2, 6+2, 8+2, 10+2 & 12+2.

So your option may be to add in 4+2 (and expect some impact) and have a hot spare, or purchase more drives

edit: clarification after re-reading q....

You'd better expand pools by the same number of disks as orginally present in the pool for consistent performance.

Here are some userful documents:

https://www.emc.com/collateral/hardware/white-papers/h8222-vnx-virtual-provisioning-wp.pdf

https://www.emc.com/collateral/white-papers/h12682-vnx-best-practices-wp.pdf

Manual will only allow to choose manually certain 6 of 7 disks, but not the RAID level

Change the disks selector to Manual then select them.

(not that I'm endorsing what you are trying !)

Expand Storage Pool wizard doesn't allow to choose RAID-level. It seems to me correct to choose 6 disks too, but it isn't specified anywhere that they will work as RAID6(4+2)

You can't change the RAID level, but you can add the disks to it (even though it says 14+2 the OK button is available).

** Before you go and click OK, please consider the advice about the potential for performance impact of doing so.

Adding a 4+2 R6 set into that tier (especially with very dense 4TB disks) will likely have a performance hit. **

Now I have no other options of expansion of disk space. If later I add 9 more disks 4000gb, it will turn out within one capacity tier will be 2 RAID6? One of 14+2 disks 2000gb, the second of 14+2 disks 4000gb

Unfortunately in these documents there is no answer to my question

No, the private Raid Groups created when you expand the pool stay the same regardless if you expand it with further drives later on.

If you go to the Properties for the Pool and look at the Tiering tab, each tier will show what the current Raid Configuration is. This is the number of disks for each raid type - ie R5 4+1 or R6 6+2. You want to expand the Tier with the multiples of the number of disks. So for a R5 tier (4+1) you would add disks in multiples of 5. For a R6 (6+2) you would add in multiples of 8.

You can not select the Raid Type as that is already set when you created the Pool.

glen