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CPG: how choose 'set size'
https://www.3parug.com/viewtopic.php?f=18&t=1206		 Page 1 of 1
Author:  nORKy [ Thu Mar 05, 2015 11:12 am ]
Post subject:  CPG: how choose 'set size'
I have a 7200 with 2 cages: 2 SSD and 11 SAS disks per cage. SSD is for AFC.
How choose the set size for a RAID 5 ?
In 1 month, we will add a cage : will I need to change de 'set size' ? how about availabilty ? I will change to cage ?

Thanks you
Author:  ADailey [ Thu Mar 05, 2015 11:35 am ]
Post subject:  Re: CPG: how choose 'set size'
What are the business requirements from an availability standpoint?

As you only have two cages you can't perform HA cage with RAID5 and would need to use HA mag. Could you consider using RAID1 with HA cage for now and then after adding the 3rd cage go to RAID5 with HA cage assuming you have the DO licensing? This would obviously reduce your capacity, but would give a higher level of HA; it all depends upon what your business requirements are.

-Aaron
Author:  nORKy [ Fri Mar 06, 2015 9:04 am ]
Post subject:  Re: CPG: how choose 'set size'
OK, thanks you
What about the set size ? What is the best pratice ?
Author:  JohnMH [ Mon Mar 09, 2015 2:23 pm ]
Post subject:  Re: CPG: how choose 'set size'
Quote:
I have a 7200 with 2 cages: 2 SSD and 11 SAS disks per cage. SSD is for AFC.


You should have an even number of disks per cage, not saying it won't work but it's not a supported configuration, so one cage should have 14 disks and the other 12.

Quote:
What about the set size ? What is the best practice ?


22 isn't ideal, 24 would have provided more choices, so you'll have a few chunklets left over, to maintain balance and minimize this I would run either of the following:-


4+1 = 4 x 4+1 =20
8+1 = 2 x 8+1 =20

You'll still access and use all the disks as the set sizes will wrap around. it's just you won't be able to access every last chunklet as eventually 2 disks will have more free and you can't form a 4+1 or 8+1 across only 2 disks. So ideally the set size you choose should divide evenly into the number of disks and the number of sets be evenly divisible by the number of controllers.
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