Adding a 600GB disk to a group with 300GB drives

[OFC_EDM]

I have a disk group that's alarming at 95.2%.

The group is made up of 300GB FC 10K drives.
And the only drive I have available to group is a 600GB FC drive.

The total capacity of the group is 14798.64GB
The current occupancy is 14092.1 GB.
55 disks in the group.

SINGLE protection.

What happens to my capacity if I add a single 600GB drive?

Will the capacity actually decrease? Since I believe the group will now reserve two virtual disks of 600GB in size instead of 300GB (which it should be doing as it currently only hadd 300GB drives in the group)

Thoughts?

[Uwe Zessin · Accepted solution]

Correct. Protection level SINGLE means: "reserve capacity of 2xSIZEOF(largest disk drive in this group)". So the desired reservation grows from 600GB to 1,200GB.

And as you have an odd number (55) of disks in the group, the EVA will 'marry' a 300GB disk with the 600GB disk (for VRAID-1 data). As far as I can tell, only 300GB can be used on the new disk drive.

So you 'win' 300GB by the additional disk drive, but 'lose' 600GB for the protection level. Unless I miss something that's -300GB.

All these are precautions if one fills (or attempts to do so) a disk group with 100% VRAID-1 data new or at some future day.

[OFC_EDM]

Thank you.

All the disk is carved as Vraid5. That would hurt a little less than your scenario with the vraid1.????

Note I have 7x600GB ungrouped in the array. Guess I could add 3x600GB disks.

Reservation would go from 600 to 1200GB.

But then I'd get 1200 from the two additional drives.

That make sense?

[Uwe Zessin]

The 'marrying' of disk drives happens when a disk group is created or as part of a disk group/ungroup operation. It is independed from whether you really use VRAID-1 or not. Same for the "protection level" - it is always calculated/ reserved, even if you would fill a disk group with 100% VRAID-0 data.

[Marcus Schack]

Well, another solution would be to create a second group of just the 600GB drives and move some of the data over there. In my experience, it has never been a "good thing" to have disk groups that have mixed disk drive size/speeds. Yes, I know HP Best Practices says you can, but it comes with a bunch of other little "gotchas".

[Peter Mattei]

Marcus

It looks like he has only one 600GB disk available.
The min # of drives you need to create a disk group is 8 ...

Cheers
Pete

[Uwe Zessin]

> Note I have 7x600GB ungrouped in the array.

Sounds like one disk is missing for a new disk group.

[Peter Mattei]

Uups, my fault. Missed that statement ...

In that case I would go for adding those 7 drives to the existing disk group.
The overall performance distribution will in most cases be better than having a large 300GB and a very small 600GB group.

Your net gain will be round 6x 600GB.

Cheers
Pete

[Marcus Schack]

Sorry, I thought he had 7 ungrouped drives plus the one he was thinking of adding to the current group. The only thing I would worry about is if Command View will then report the proper capacities in the mixed group. Going way back, it used to be an issue getting accurate capacity numbers in the disk group.

[OFC_EDM]

Added the 7 x 600Gb drives to the group.
Currently levelling but all looks good.

Thanks for the feedback.