EVA Disk Group Math

[Adam Garsha]

I am trying to figure out formulas so that I can predict vdisk space available when adding drives.

Here is a CSV formatted view of a subset of our Disk Groups (that are fully leveled):

Disk Group,Total,VRAID0 Available,VRAID1 Available,VRAID5 Available,Occupancy,%-Occupancy,Disks,146GB 10K,72GB 10K,72GB 15K
Applications,3544.93,1180.1,595.11,944.2,2340.7,66,42,18,24,0
Disk Pool A,1902.94,274.63,137.35,202.03,1499.15,78.8,28,8,20,0

All my disk groups are set to a protection level of 2. My question is this, why don't the numbers add up? For example, "Applications" has 3544.93GB Total and 2340.7GB Occupied. So, I would think that I should have <3544.93GB - 4x146GB(for protection level 2) - 2340.7GB> available for VRAID0, correct?

But, I get:

3544.93-4x146-2340.7 = 620.93GB, but the EVA reports 1180.1GB available for VRAID0. Can you help me get my math right? Am I counting incorrectly?

Online help says "Occupancy" is space set aside for vdisks, it does not say that it includes the protection disks as well.

Thanks.

[Andrzej Kowalik]

Adam,
if you have more disk groups, you loose 4 disks for each group.
regards,
Andrzej

[Adam Garsha]

Does "Total Capacity" as displayed by the EVA for a Disk Group already take into account the disk space put aside for "protection level"?

[Adam Garsha]

Does anyone have formulas handy for Disk Group space calculation where the inputs are effective size and number of disks, Occupancy, and protection level, and the output is "Total Capacity", and space available for each of VRAID levels?

[Uwe Zessin · Accepted solution]

Please note that the disk sizes (146 + 72) are specified in hardware (base 10) GigaBytes, while the EVA output is in software (base 2) GigaBytes. Take a look at the disk drive's property page to see what the EVA uses as a space calculation.

The online help is right - Occupancy is raw capacity used by all virtual disks, it does not include protection space. There are no 'protection disk' - the capacity is levelled over all disks drives in the disk group.

I suggest you take a look at your current values and then lower the protection level to 1 or even 0. Have no fear! It won't do any harm to your system! Then check the values again.

Oh, and watch out if you get offered any sizers or formulas. I have verified a few and some of them confuse hard- and software GigaBytes, some include a silent 5% space reservation in the disk group.

[Mark Poeschl_2]

You are correct - 'Total' capacity DOES already reflect loss of the capacity set aside for sparing protection. You may also be forgetting about the 7% hardware 'tax' that you have to allow for when sizing disk arrays. When a H/W vendor sells you a 72 GB disk it holds 72,000,000,000 bytes. Nearly all S/W and storage calculations think 72 GB is 72*1024*1024*1024 = 77,309,411,328 - a discrepancy of about 7%. The best way to calculate your real capacity is to click on one disk drive of each type and see what Command View gets as the drive capacity.
For example, the 300 GB drives in my EVA show a 'formatted capacity' of 279.39 GB - that darn 7% again.

Then multiply by the amount of drives of each type and subtract the appropriate amount for sparing protection. That should give you an accurate number to start from.

[Mahesh1]

Hi, for more information refer page 9 and 10.
Regards
Mahesh Gowda

[Frank Cincotta_1]

Here is an older EVA sizer, that allows you to put in how many disks, vraid complement and so on and come out with space calculations.

[Uwe Zessin]

That is one of the sizers which uses HW GB at some columns, but SW GB at some other columns.

E.g. on sheet "EVA Configuration A", column "E" is HW GB (although the EVA does not display HW GB) and column "I" is SW GB.