Creating a second disk group from an existing disk group.

[Gene Kenny]

Hi

I have one disk group with 28 drives. I would like to remove eight drives from the existing disk group and create a second disk group.

A. Will it be ok to have a disk group with 20 drives, when the best practice says disk groups should have a multiple of 8 drives?

B. When ungrouping the drive how long should I wait to remove the next drive, how long will it take for the existing disk group to normalize after a drive is removed?

Thanks

[IBaltay · Accepted solution]

Hi,
from the performance point of view this is not a good idea, because the following is valid here: the more disks in the disk group, the more performance.

a) it is better to add/remove disks in 8 because of the RSS
b) after the ungroup of the single drive (when it appears in the ungroupped group), you can continue to ungroup other drives. The leveling process is a separate process and you dont need to wait for the leveling to finish between the ungrouping

[Steven Clementi]

Gene:

1. What model EVA do you have? and how many shelves?


"A. Will it be ok to have a disk group with 20 drives, when the best practice says disk groups should have a multiple of 8 drives?"

The simple answer is "yes". Sure, the disk group is trying to protect your data by splitting the group up in sets of 8 drives.

Right now you probably have:

1 RSS's of 8 disks and
2 RSS's of 10 disks each.

The EVA automatically selects the disks it creates a RSS out of so as you remove drives, your RSS's change.

You will end up with:

1 RSS's of 8 disks and
2 RSS's of 6 disks each (in the original disk group).

RSS's will always be as small as 6 disks or as big as 11 disks. The EVA tries to keep them at the number 8, which is where the best practice comes in.


Steven

[Timothy Cusson]

A. Will it be ok to have a disk group with 20 drives, when the best practice says disk groups should have a multiple of 8 drives?

A. Yes it is OK, but you will not be following best practice. As Steve said the result will be...
DiskGroup1, 20 disks
RSS-1 8 Disks
RSS-2 6 Disks
RSS-3 6 Disks

DiskGroup1, 8 disks
RSS-4 8 Disks

B. When ungrouping the drive how long should I wait to remove the next drive, how long will it take for the existing disk group to normalize after a drive is removed?

"Best Practice" is to only ungroup one disk at a time, see the attached HP Customer Advisory. Ungrouping is one process, then levelling is another. The time required is dependent on the amount of data contained on each drive, ie: it's occupancy. Also, these are background process', so if the EVA is busy doing disk I/O, the ungrouping will take longer.

Regards,
Tim