DukeR1337

208 Posts

2731

November 21st, 2017 05:00

Capacity questions when expanding existing pools (RAID5, RAID10)

Hi all

I have a customer with a VNX 5300 which is configured with 1 DPE and 2 DAE. DPE is equiped with 15x 300 GB disks, DAE 1 with 600 Disks and DAE 3 with 600 DB disks as well. BUT DAE 2 resp. its disks are not assigned yet.

Now is it possible to expand Pool 1 which contains 15x 300 GB Disks in a Raid (4+1) configuration with the additional 15x 600 GB Disks from DAE 2 with a Raid (4+1) as well WITHOUT capacity lost due to the different disk capacity? In my opinion it should be possible because the new disks are own raid groups and deliver the full capacity to the pool. I just need confirmation about this becuase you guys now you can not remove the disks afterwards

PS: I have two other questions. 1) What exactly happens when I use Raid 5 (8+1) with 15 Disks? Do I loose capacity or performance? I guess so but how exactly do I calcucalte this loss? 2) Why is a RAID 1/0 (10) built with 4+4 within EMC and not 2+2? As far as i understand a RAID 10 is a RAID 0 striped about two RAID 1 so in theory it should be possible to use it with 4 disks instead of 8. What happens if I choose RAID 10 (4+4) but only add 4 disks instead of 8?

Thanks for all your help which I really appreciate

Duke

Responses(8)

kelleg

4.5K Posts

0

November 30th, 2017 09:00

When you add additional disks to an existing Pool, the new disks (600GB) will retain their capacity. The problem with disk capacity is when you try to make a raid group using both 300 and 600 disks - the available capacity will always be that of the smallest disk. Since your Pool is already using 300GB disks, adding the 600GB disks will create new private raid groups that do not include the 300GB disks.

If the existing raid type/width is R5 4+1, you must use that with the 600GB disks. Add the 600GB disks in multiples of 5 (4+1). If you have 16 disks then add only 15 (three sets of 4+1).

1) What exactly happens when I use Raid 5 (8+1) with 15 Disks? Do I loose capacity or performance? I guess so but how exactly do I calcucalte this loss?

With 16 disks using 8+1, you will end up with one private raid group as 8+1 and the second private raid group as a 6+1 - this will lead to lower performance as the stripe width will be different - EMC recommends that you not do that.

2) Why is a RAID 1/0 (10) built with 4+4 within EMC and not 2+2? As far as i understand a RAID 10 is a RAID 0 striped about two RAID 1 so in theory it should be possible to use it with 4 disks instead of 8. What happens if I choose RAID 10 (4+4) but only add 4 disks instead of 8?

You can use a minimum of two disks on VNX1 arrays (1+1) if you want, EMC recommends using 8 (4+4) for best performance. You will get a warning about the recommended 8 disks when you create a new Pool using 4 (2+2) disks, but you can ignore that if you want.

I would recommend that you review the best practices guide for VNX1 - that contains a lot of the information that you're asking.

http://www.ndm.net/emcstore/pdf/vnx/h8268-vnx-block-best-practices.pdf

glen

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DukeR1337

208 Posts

0

November 24th, 2017 04:00

No one?

G

gleo219

7 Posts

1

November 28th, 2017 03:00

Why don't you create a new pool with the 600 GB drives you can migrate luns between the pools if required.

Answer to the first of the other two questions you can't create Raid 5 (8+1) with 15 disks, it will only use disk increments of 9. So it would be 9 disks 18 disks or 27 disks etc

Useful document to read

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

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DukeR1337

208 Posts

0

November 30th, 2017 04:00

Hi

Thanks for your response. Well there is a certain reason to not create a new pool: mainly performance. So still my question is am I going to loose the new disks capacity when creating 3x RAID 5 (4+1) RAID sets with 600 GB disks within and existing pool with RAID 5 (4+1) with 300 GB disks?