EVA4000 mixing disc size in one group

[Piotr Staszewski]

Hi,

we are out of space on our eva4000 :) (2C2D). We would like to add new encl (migrate to 2C3D) with 2x300GB drives. We already have group of 18x143GB drives which we would like to extend.

I know that after adding 300GB of drives we will gain only 2x143GB due to RSS and other issues, and that mixing drive sizes is not recommended by HP.

But what will happen if we will add more 300GB drives to this group? IMHO they will be more loaded (I/O) than 143GB as they will hold more data, anything else?

Or maybe I shouldn't use 300GB drives and just add 143GB... and someday replace all to bigger size? (this scenario seems to be to expensive)

[IBaltay]

Hi,
here are the eva best practices. It is for 4000/4100, 6000/6100 and 8000/8100, but the principles remained the same also for the 4400/6400/8400:
http://h20195.www2.hp.com/V2/GetPDF.aspx/4AA0-2787ENW.pdf

[Víctor Cespón]

Adding 2 x 300 GB you would not even gain 2 x 143GB. Yes, the "disk failure protection" will go from 2x146 to 2x300, so you add 600 and lose 300. But the big disks will never be 100% used so you'll not have 300 GB more available.

The more 300 GB you add to the disk group, the more space will be used in them. This is due to the basic fact that RAID 5 information has to be stored on 5 different disks. The controllers try to store as much data as possible on the big disks without breaking that rule.

I had to make a drawing to explain this to some customers.

[Piotr Staszewski]

cite: " The more 300 GB you add to the disk group, the more space will be used in them. This is due to the basic fact that RAID 5 information has to be stored on 5 different disks. The controllers try to store as much data as possible on the big disks without breaking that rule."

the only issue is - we use RAID1 in this group ;)

[Uwe Zessin]

Then it is less of a problem as long as those two disk drives are 'married' (and kept as a couple) for VRAID-1 data.

Oh, by the way...
Last time I checked, you need a minimum of 4 disk drives in an EVA-4000 disk drive enclosure.

[Piotr Staszewski]

hmm... thanks for pointing it. I've asked my hp supplier for offer - new encl with minimum drives to make it running and he proposed two drives.

[Víctor Cespón]

the only issue is - we use RAID1 in this group ;)

It applies too, for performance reasons the EVA will add each 300 GB disk to a different RSS, so it will be "married" to a 146 GB disk and only 146 GB of that disk will be used.

It's not until you have many 300 GB disks that they will be joined in pairs of the same size.