EVA Disk Group Size

[Richard_Maxwell]

Hi,

We have an EVA 4400. I noticed that the default disk group for the EVA had 12 disks in it and there were several ungrouped disks elsewhere in the array. So, I added them to the group, but it only let me add three. So, now, it has 15 disks in the group. Apparently, it's only licensed for up to 5 TB in a disk group, so it won't let me add more. I read elsewhere in the forum that you should add disks to groups in multiples of eight. So, I'm wondering if I should have removed four of the disks to make eight? Or, do the advantages outweigh the drawbacks of not using multiples of eight? Also, could someone explain what the drawbacks are of using a group not divisible by eight? Is it a reliability issue, or does it actually slow the array down? In other words, which is a better number of disks in the group: eight, 12, or 15?

Thanks,

Richard

[Cali]

1st Rule:
"Use of a single EVA diskgroup, whenever possible, is adequate for all storage optimization types (cost, performance, capacity)."
2nd Rule:
"Use Disks in disk groups in multiples of eight, to Enable the EVA to optimize the distribution of disks in the Redundancy Storage Set (RSS)."

"Disk groups are independent protection domains, as such, all data redundancy information and reconstruction space must be contained within the disk group. Unlike traditional RAID 5 arrays, where the stripe depth and redundancy set are the same, the EVA supports many disks in the stripe set (a disk group), but the redundancy set (RSS) is always 6 to 11 disks."

So 8 is the Middle between Maximum Parity (5+1) and Minimum Parity (10+1).
The EVA divides the number of Disks in the Group and than choose the possible RSS Sets for you.

15 Disks are a very low Number for an EVA, I would not create a second Group with less than 50-100 Disks in one Group.