CPG set size

[Radeon]

Good day to all.
I ask for your help, please explain if there is a connection between cpg set size and the number of physical disks.
I have an array with cage, a total of 20 physical disks installed. By default, when initializing the cpg set size system, select 6 + 2 and availability Magazine.
Do I understand correctly that 8 + 2 is optimal for 20 physical disks?

Thank you all in advance for your answers.

[MammaGutt]

Set size = the number of disks/chunklets involved in creating an LD.

If you have 20 drives, a set size of 10 will allow you to write two full stripes hitting all drives one time*.


*assuming a lot of factors like not reusing free capacity in existing LDs and round robin hitting each drive once.


The driving factor for most people is higher set size = lower RAID capacity overhead, but if you play with HA cage you quickly get a lot of cages....

[Radeon]

Thank you MammaGutt.
Then the question is, is it possible to change the cpg set size ratio on the current array? Will this action lead to irreversible consequences?

[MammaGutt]

Thank you MammaGutt.
Then the question is, is it possible to change the cpg set size ratio on the current array? Will this action lead to irreversible consequences?

The quick and dirty is to change the set size on the CPG and run tunesys.

The other way is to create a new CPG and tune each volume.

For the last one you need at least the size of the largest volume in free space. For the first one you could manage with less but always good to have 15-20% free capacity.

[Radeon]

Thank you MammaGutt.
Then the question is, is it possible to change the cpg set size ratio on the current array? Will this action lead to irreversible consequences?

The quick and dirty is to change the set size on the CPG and run tunesys.

The other way is to create a new CPG and tune each volume.

For the last one you need at least the size of the largest volume in free space. For the first one you could manage with less but always good to have 15-20% free capacity.

Are you saying to have 15-20% headroom and not actually use 4 disks? But isn't it more efficient to use the entire volume of the group?
MammaGutt, is it possible to lose data when changing cpg set size?
and do you think it would be better for me to switch to cpg set size 8 + 2 or stay at 6 + 2

[MammaGutt]

Thank you MammaGutt.
Then the question is, is it possible to change the cpg set size ratio on the current array? Will this action lead to irreversible consequences?

The quick and dirty is to change the set size on the CPG and run tunesys.

The other way is to create a new CPG and tune each volume.

For the last one you need at least the size of the largest volume in free space. For the first one you could manage with less but always good to have 15-20% free capacity.

Are you saying to have 15-20% headroom and not actually use 4 disks? But isn't it more efficient to use the entire volume of the group?
MammaGutt, is it possible to lose data when changing cpg set size?
and do you think it would be better for me to switch to cpg set size 8 + 2 or stay at 6 + 2

The 3PAR will use all disks. They har divided into chunklets. So with set size 8, it goes "something" like this
LD1 disk 1,2,3,4,5,6,7,8
LD2 disk 9,10,11,12,13,14,15,16
LD3 disk 17,18,19,20,1,2,3,4
LD4 disk 5,6,7,8,9,10,11,12

Not possible to lose data.

It will give you a little more effective space.