EVA's & Disk addition - net computations

[S.Martin]

I am contrasting and comparing disk addition decisions for my customer.

I need to find a document that discusses the space loss for parity, overhead, and other net losses that occur with a SAN.

For instance, when you add a 146GB, net space availability is shown to be about 136GB. I believe that this depends on sparring, number of disks in disk group, etc. but want to know for sure.

Please let me know if you are aware of a document that lays this out.

Thanks!
Suzanne ...................

[Rob Leadbeater]

Hi Suzanne,

Not a document, but the StorageWorks sizing tool available here is very useful for this sort of thing:

http://h30144.www3.hp.com/SWD%20Sizer%20Web%20Site/

To explain your example, disks are sold in metric GB ie 1,000,000,000 bytes, whereas most storage systems actually display things in binary GB (GiB) 1,073,741,824 bytes.

Therefore you convert between the two with his multiplier:

1000000000 / 1073741824 = 0.9313

146 x 0.9313 = 135.9698

Hope this helps,

Regards,

Rob

[Hasan Atasoy]

hi suzanne;

your disk size is 146000000000 byte.
146000000000/(1024*1024*1024)=136GB.


Hasan.

[S.Martin]

Yes, thanks.

But to clarify, I was referring to the available disk space difference computations in configuring vraid1 as opposed to vraid5. i.e. parity loss, redundancy loss, etc.

For instance, if I have a diskgroup with 24 146GB drives. I have a project request for quote coming along needing 'X' GB's of space. I need to be able to accurately define how many drives they need to purchase to provide the required space. This includes net loss for parity and redundancy to accommodate for vraid1 and vraid5 needs.

In the past, I have noted that it depends on the number of drives in disk group, the size of those drives, etc.

I would like to be more sure of my answers to project owners to protect my storage resources.

Ideas, docs, refernces?

Suzanne

[Hasan Atasoy]

hi suzanne ;

you should look at thread

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=637957


Hasan.

[Richard Tengdin · Accepted solution]

The basics:
1. Nameplate capacity on drives is in powers of 10 (HW GB), EVA allocates space in powers of 2 (SW GB).
2. Best Practice is use no more than 90% occupancy.
3. EVA RAID 1 overhead is 100% (need 2 GB free space to create a 1 GB VDisk)
4. EVA RAID5 overhead is 25% (need 1.25 GB free space to create a 1 GB VDisk)
5. Sparing is at the Disk Group level, if you add a new drive you will see the SW capacity
6. Add drives in Pairs to see an increase in available VRAID1 space, unpaired drives do not hold any VRAID1 data.

Here is a table of Disks per TB by RAID level at 90% occupancy, (after #1 and #2 are accounted for):

I have entered it comma separted, you can paste it into Excel and it should parse correctly:

HW, SW,90%, R0, R5, R1
146.8,136.7,123.1,8.4,10.5,16.7
300.0,279.4,251.5,4.1,5.2,8.2
500.1,465.8,419.2,2.5,3.2,4.9,
750.2,698.6,628.8,1.7,2.2,3.3

Remember that the Spare Space calculation is based on the largest drive in the group, so adding two 300GB drives to a group of 146s will not add 560 GB of free space. At Single protection you would get about 280GB back. At Double you would *loose* an additional 560 GB of free space!

[Rob Leadbeater]

Hi,

A couple of corrections...

> 2. Best Practice is use no more than 90% occupancy.

That best practice is now out of date. Current best practice is to leave 5GB free in the disk group.

> 4. EVA RAID5 overhead is 25% (need 1.25 GB free space to create a 1 GB VDisk)

I thought VRaid5 overhead was 20%, not 25%.

Cheers,

Rob

[S.Martin]

These are great answers! Thanks all! I need to justify / validate the purchase 300's and NOT 146 GB drives .. I don't even imagine that HP would recommend a purchase of 146's at this point.

I have BC & CA so I am proposing for the purchase of 300's snapcloning stuff gradually from the 146 DG over to the new faster 300 DG.

additional comments?

[Hasan Atasoy]

hi suzanne;

you also consider iopersec that storage can do besides capacity.

you will buy half of the required disk if you choose 300Gb. it will decrease also storage iopersec capacity.


Hasan.

please also assign points if the information is helpfull for you.

[S.Martin]

You are all great! thanks!