How to Estimate EVA (5K/8K) Usable Capacity

[Alzhy]

We practically use VRAID5 all the time and I have come up with a formula to estimate net usable storage in a Diskgroup as follows:

with N = number of Drives in the DiskGroup
DSIZE = size of each Disk

All VRAID5 = {(N-2) x DSIZE)}*.80
All VRAID10 = {(N-2) x DSIZE)}*.50

Is this correct?

Or does HP provide a more accurate guide/formula?

Thanks.

[Uwe Zessin · Accepted solution]

Looks OK to me if you use a consistent size (either hardware or software gigabytes).

Over the years there have been all kinds of formulas and sizers, but I have not been happy with what I have seen (well, OK, I am not easy to please and I have not looked at current tools ;-)

Things to watch for:
- tell the difference between hardware and software gigabytes
- what 'protection level' is assumed, if any?
- how much implicity free space in a disk group is assumed? Some sizers implicitly reserve 5%.

I have developed a 'forward sizer': user specifies required capacity and VRAID-level. I use that raw capacity, multiply it by 1.001 for some meta-data overhead and divide it by disk capacity in software gigabytes (SWGB) to get an estimate how many disk drives are required at least.

Then I check for at least 10 SWGB for free space (yes, I know some HP documents say 5), maybe add a disk drive to get an even number and finally add the number of disks to satisfy the 'protection level'.

With the number of disks I have an estimate for the possible number of I/Os the disk group can deliver on a sunny day ;-)

[Rob Leadbeater]

Hi,

That looks good, assuming that you've got the protection level of the disk group set to Single. If it were double then your N-2 would change to N-4.

Cheers,

Rob

[Tim Nelson]

Doesn't command view give you the estimates for raid 0,0+1 and r5 ?

[Alzhy]

Tim,

The question really was to get an estimate of what A Diskgroup will net you as far as usable space given a set of disks/enclosures... i.e. I want to buy 4 enclosure fulls of 500GB FATA Drives (48 Drives) -- How much usable will I have assuming ALL Vdisks will be VRAID5 and for Single or Double Redunancy...

[Tim Nelson]

Got it..

Might want to keep in mind also that a 500GB drive is not really 500GB.. market number vs formated usable space..

[Alzhy]

Yep..

We use True Gigbytes of 1024*1024*1024 bytes.

Vendors do continue to use "Marketing bytes" in terms of billion of bytes... We've been bitten by this in tha past and I've heard others have gone to litigation as sometimes Vendors would not stipulate clearly that 1 Gb is 1,000,000,000 bytes.

[Uwe Zessin]

> Doesn't command view give you the estimates for raid 0,0+1 and r5 ?

Not always. It is better to say that it gives the max. available value for the next virtual disk. If there is more space available in the disk group, CV just shows 2047.99GB.

And it would not help you if you had to size a box _before_ spending money ;-)


> "Marketing bytes"

I call it the difference between "hardware gigabytes", what is printed on the disk drive itself which is based on 1000^3 and "software gigabytes", what Command View EVA shows, which is based on 1024^3.


E.g. BD3005823 "300GB" disk drive has 585,937,500 blocks, according to SSSU.

585,937,500 * 512 = 300,000,000,000 (whow, exactly 300 HWGB)


Command View EVA shows: 279.39 SWGB

585,937,500 * 512 / 1024 / 1024 / 1024 = 279.396484375


It is the same amount of storage, but a different base.
Or would anyone claim that 101(2) is "more" than 5(10)?


> formated usable space..

In the early days of the EVA, the 7% (1/1.204/1.024/1.024) difference was explained as 'virtualization overhead'.

Yes, there is some overhead for tagging the disk drives and the mapping tables, but it is not _that_ high.

[Peter Mattei]

I suggest you use the HP StorageWorks Sizer. Download it from here http://www.hp.com/go/nsssizer

This is the official HP tool to size all kinds of HP storage solutions inclusding EVAs, XPs, NAS, MAS, Backup etc.

When started click on item
- 4. Size an XP/EVA...
- Launch EVA Calculator
Now you can start selecting the EVA type create disk groups with various sizes, disks etc.

Note that you can selct under Preferences (top right) if you want 1k to be 1000 or 1024 bytes.

Cheers
Peter

[Mark...]

Hi All,
Ahh "salesmans" size Vs real size, always a bone of contention and lots of big maths calculations flying round here as well !
The simple method I know of is for every 36Gb of unformated then remove 2.5.
36.4 unfor - 2.5 = 33.9. Actual = 33.91 = close enough for me.
All you need to do now is scale up
300 / 36.4 = 8.24 * 2.5 = 20.6
therefore 300 - 20.6 = 279.6
Again, not foolproof but it makes it easy enough to work out. All disks same size then multiply by X disks. Different disks sizes them multiple diff sizes then add to gether = capacity.
I think we all know the sparing bit so then just remove x2 or x4 of largest capacity drive in disk group.

Mark...