CPG & Raid Set Size

[J-Philippe]

Hi to the experts, I'm looking for a Best Pratices guides regarding Raid Set Size.

What should be the best to do if :

Adding x6 "3.8 TB" drive to a 3PAR with x12 "1.9 TB" within one CPG RAID 6 with a set size of 12 (10+2) 

Reduce the set size ? What will be the impact if I let the raid set size of 12 with now 18 drives instead 12 ?

If any tips and BP guide please let me know.

Best Regards.

Jean-Philippe

[sbhat09]

Hello Jean,

Here is the document you may find it useful - https://support.hpe.com/hpesc/public/docDisplay?docId=c04204225 

Regards,

Srinivas Bhat

[J-Philippe]

Thank you, the Admin Guide provide some tips but not enough from my opinion.

I'm looking for more deep information regarding Raid set size.

For instance (tell me if I'm correct or not) working on a RAID6 Set size of 12 implies to use 12 chunklet. (10 data + 2 parity)
- Data is stored on Logical Disk. An LD is collection of "n" Chunklet depending the set size.
- In this situation the LD size is 12 GB
- Having 12 disks of 1.9TB in the array make that the Set size and disks are aligned : 1 chunklet of 1Gb on each disk.
- The LD relies on the 12 physical disks.
- Adding 6 more disks of 3.8TB in the array (12+6) and not changing the set size mean that LD are not aligned with disks.
- How the 3PAR can manage this ? Will the new disk be capped at 1.9TB (like in a legacy proliant RAID) ?
- How the 3PAR will decide when to use LD made of raid set of 12 based on new and "old" disks ?

Thanks for your help.
Jean-Philippe.

[sbhat09]

Hi Jean,

Scenario: You have 12 disks. Now adding 6 disks of identical capacity and admitting to the same CPG. Set size continues to be 12.

How it works is: After tunesys, existing data will be balanced to all 18 disks. When new LD forms, it picks 1 chunklet each from disk 0 to 11 (first 12 disks). For the next LD it picks chunklets from 11 to 17 and 0 to 5 (12 disks). subsequent LD will pick from 6 to 17 disks. The cycle continues so that new LDs equally utilize all 18 disks.

Hope I answered both of your queries.

Regards,

Srinivas Bhat

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[J-Philippe]

Hi Srinivas, thank you for your answer, it help me to better understand.

One question remains about adding disks with more capacity. Maybe it is not advise but neither forbidden. 
The disk usage will not be the same regarding LD pick from 1.9TB drive and 3.8TB, I guess in such situation I lost some space effiency because 1.9 TB drive will be full before the 3.9 TB if not modifying the Raid set size. 

Best Regards,

Jean-Philippe.

[Sheldon Smith · Accepted solution]

Yes. And, since you have a set size of 12, then you need 12 disks to get the chunklets each time it needs to expand by creating a new RAIDset.
When the 12 smaller disks fill back up and you are left with only the 6 larger ones, the CPG won't be able to expand any further.

[J-Philippe]

Hi Sheldon,

Thank you that's clear.

Best Regards,

Jean-Philippe.