Pools and underlying RAID groups size

Yes, you seem to be right. There's no much difference in RAID Group and Pool. There functionality and performance will differ. Plus the utilization will also differ. In terms of number of disks adding to a RAID Group or Pool (with same size comparision), if you will add more number of disks, will get more space, in case of 7+1 in RAID5 the parity overhead will be lesser compared to 4+1. You can search "EMC VNX Thin Provisioning" on google.

Thnx

Rakesh

If I add 20 disks to a pool based on 8+1 RAID5 groups,

EMC recommends (4+1)RAID 5, (6+2)RAID 6, (4+4)RAID 8.

Sorry, it doesn't answer my original question.

Yes – a pool has a preferred raidgroup size per raid level (tier)

For R5 / R6 you have a choice when first adding a tier to a pool

Yes – if you add a number of disks that aren’t a multiple of the preferred RG size we create smaller RG’s for the last 1 or 2 RGS

I don’t think the rules for that are documented externally.

We basically do what makes sense

if you add 7 disks on a 41R5 pool you will get a 61R5 – if you add 8 you get a 41R1 and a 21R5

Rainer,

In my case I have underlying RAID groups LARGER than they are supposed to be. Not 8+1 but 9+1. Of course, if you have 20 disks and create 2 x 9 disks RAID5, you have only 2 disks remaining and can't create an additional RAID5 group using them. So it seems quite logical to increase the size of two 8+1 RAID5 groups making them 9+1 RAID5.

However, it makes the system behavior very unpredictable. For example, what would happen if I add two new disks to the pool of 20 disks? It's total size will be 22 disks, which equals to 2x9 + 1x4, and the system CAN create an additional RAID5 group with those disks. What will happen in reality? Will the pool be repartitioned to replace 2x10d RAID5 with 2x9d RAID5 + 1x4d RAID5? If so, the actual increase in capacity will be less than expected (capacity of one of the disks will be used for supporting the new RAID5 group parity).

I'm just trying to understand how the system behaves in different situations and how to plan its upgrade correctly. It's not always possible to buy any desired number of additional disks and DAEs, you know. Budgets are always limited.

yes if you add 20 disks on a 8+1R5 pool the system will create 2x 9+1R5

Once internal raidgroups are created for a pool they will NEVER change.

if you add disks they will be used for new internal raidgroups

so in your example if you try add another 2 disks it wont allow you - since the minimum drive count for R5 is 3

Hi Rainer,

Does this mean that we have to choose multiples of 3 or they can be like this 3+1, 5+1, 7+1, 9+1 (with R33, I mean multiples of 3 or should be Odd numbers) ?

Thnx

Rakesh

if you add 7 disks on a 41R5 pool you will get a 61R5 – if you add 8 you get a 41R1 and a 21R5

Could you please elaborate the underlined sentence...

Thanx

Rakesh

rakesh.pandey@zensar.in wrote: Hi Rainer, Does this mean that we have to choose multiples of 3 or they can be like this 3+1, 5+1, 7+1, 9+1 (with R33, I mean multiples of 3 or should be Odd numbers) ? Thnx Rakesh

I dont understand the question

There are two preferred RG sizes for a SAS Tier in a pool - either 4+1R5 or 8+1R5

Which one you want you decide when you add that Tier to a pool for the first time.

If the number of disks you are adding isnt fully divisble by that disk multiplier (5 or 9) then the VNX adjusts the RG size for one or two RG's that are added so that all the disks you asked are used.

reply by EMail seems to remove all the plus signs

what I meant:

if you add 7 disks on a 4+1R5 pool you will get one 6+1R5

if you add 8 disks on a 4+1R5 pool you get one 4+1R5 plus one 2+1R5

Does this mean that you can expand an original R5 storage pool by adding a minimum of 3 drives to it, regardless of the initial configuration - either R5 4+1 or 8+1, or whatever it might have been? I was under the impression, that the best practice would be to add the same number of drives of the same type as originally configured.

yes

I got the second point. My understanding is this (If 7 disks are added to a pool with 4+1=5 config, the system will pick 2 disks automatically ?) with (if you add 7 disks on a 4+1R5 pool you will get one 6+1R5)

This is what you meant I believe.

Thanx

Rakesh

just because you can, does not mean you should