HPE EVA Storage

Different Disks (Size) in one EVA Diskgroup?

 
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Christian Reiter
Frequent Advisor

Different Disks (Size) in one EVA Diskgroup?

Hi!

Is it possible to have disks of different sizes (For exampel 8x146 GB and 4x300 GB) in one EVA diskgroup?

What are possible drawbacks of such a config?

christian
6 REPLIES 6
Ivan Ferreira
Honored Contributor
Solution

Re: Different Disks (Size) in one EVA Diskgroup?

Yes, its possible.

See also:

http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=1114684
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=990097
Por que hacerlo dificil si es posible hacerlo facil? - Why do it the hard way, when you can do it the easy way?
Mike Bollman_1
Respected Contributor

Re: Different Disks (Size) in one EVA Diskgroup?

You can mix different size disks in the same group, but you cannot mix different types.

Meaning no mixing of FC and FATA drives.

However, if you mix 146GB drives with 300GB drives you will lose some efficiency. Lets say you have a group of 20 - 300GB drives and single sparing. Then you create a VRAID1 LUN (say 1000GB) in that disk group. You now have used up the capacity of a little over 3 spindles. However, keep in mind that since your LUN is VRAID 1 and you have single sparing enabled the EVA will drop 2 spindles worth of capacity for sparing. *Note all spindles are in use, you just lose the capacity of 600GB* If you were to add then say 10 - 146GB drives to this group you would have another raw capacity of 1.5TB. The issue is that the EVA will want to spread the data (via a load level operation) across the new spindles along with the "reserved" sparing space. So now you will end up with more of the space on the new drives used by sparing (relative to capacity) than the existing 300GB drives (20GB each spindle based on 600GB and 30 spindles total). So your thinking whats the big deal here. The issue is over time the 146GB drives would fill faster than the 300GB drives as a percent of capacity. This would in effect make them "hot" in terms of relative I/O per spindle.

Someone correct me if I am wrong here, but I think the EVA just load-levels based on capacity and not on I/O loads.
Amar_Joshi
Honored Contributor

Re: Different Disks (Size) in one EVA Diskgroup?

My $1m suggestion. When mixing the disk sizes, don't configure the Disk Protection level, leave it to "none" and keep a good amount of space free. That will prevent you from reserving too big capacity.

** Keep enough free space, if any of the disks fails (higher or lower capacity), you should be covered.!!!
Uwe Zessin
Honored Contributor

Re: Different Disks (Size) in one EVA Diskgroup?

> The issue is over time the 146GB drives would fill faster than
> the 300GB drives as a percent of capacity.

It is the other way round:
The EVA checks the fill percent of a disk group and applies the same value to each disk drive. So all disk drives will be filled with the same percent value. That means a 300GB disk drive will store about twice as much data as a 146 GByte disk drive.

> Someone correct me if I am wrong here,
> but I think the EVA just load-levels based on capacity
> and not on I/O loads.

Talked to an insider last year:
The EVA does check the I/O loads on the disk drives once an hour and might relevel.

One more undocumented feature...

--

> and keep a good amount of space free.
> ** Keep enough free space

Hmmm. But how do you define a 'good amount' and 'enough' ?

I think the so-called 'protection level' does it for you quite well.

On top of that I leave 10 GigaBytes of raw space free in the disk group unless BC or CA is being used.
.
L. B. Andersen
Advisor

Re: Different Disks (Size) in one EVA Diskgroup?

The way I understand it, the VSC code (starting from vcs3) will use the actual disk sizes to calculate how much data to put on each individual drive in order for all drives to fill at the same pace. Thus a 300GB drive will contain 4 times the amount of segments as will a 72GB drive. This works great for utilizing capacity, but may be less recomendable when configuring for performance, as the large drive potentially will recieve 4 times the number of io's than the small drives, in average, given the fact that it contains 4 times the number of segments.

Another thing to think about is sparring. If you have configured for double sparing, adding 4*300GB drives in a 146GB group, will only give you 4*146GB capacity, due to the fact that 4 times the largest dirve in the group will be reserved for spare.

Correct me if I am wrong.
jason andrade
Frequent Advisor

Re: Different Disks (Size) in one EVA Diskgroup?

i don't think you're wrong but why would people configure double sparing instead of single sparing ?

single sparing is already reserving twice the capacity of the largest disk - so you can preserve your VRAID1s..

double sparing would presumably allow you to deal with 4 disk failures (over a longer period of time..)

-j