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Different disk types in a RAID 1+0 array?

Petter Smart
Occasional Visitor

Different disk types in a RAID 1+0 array?

I have a RAID 1+0 array set with two 18GB disks, but now i'm out of space on this array. So I've decided to buy two 72 GB disks to expand this array.

But when consulting someone about this ha told me that these two disks couldn't be added to my array since it was a mirror set.
But after reading about RAID 1+0 that makes no sense.

I can't se any restrictions on using diffrent sized disks in the mirror set that later wil be striped with the eksistent mirror set.

Can anyone tell me whats possible?
Can I set up my array like the picture shows? And if that's the case, will it give me 18+72 = 90Gb in that array?
5 REPLIES
Uwe Zessin
Honored Contributor

Re: Different disk types in a RAID 1+0 array?

Nice picture, Petter, but I am afraid it will not work the way you like it. You don't say what array controller you are using, but all those that I am aware of use fixed formulas to calculate where to locate the data blocks. In that case the total usable space is limited by the smallest disk in the 'RAID group'.

I have attached a small 'picture' in which I try to illustrate this.
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Mike Naime
Honored Contributor

Re: Different disk types in a RAID 1+0 array?

Peter:

The short answer is NO, you cannot grow your LUN like that. (Not without loosing ALL of the data)

If you have an HSG80 based storage array, you can stripe mirrorsets. BUT, I believe that you will loose all of the existing data on the LUN when you make the stripeset of the mirrorsets. (I have not tried this myself)

While you can make a mirrorset of an 18GB and a larger drive, the usable space will remain the initial size of the mirrorset when it was initialized (Formatted).

Mike
VMS SAN mechanic
Arjo de Bruin
Frequent Advisor

Re: Different disk types in a RAID 1+0 array?

Hi,

I think it depents on your Raid controller. With an Smart Array 5300 for instance it's also posible to make different RAID's in one Array. The only problem i think is performance because the Smart Array wants to level the data over all disks... The 72 G disk will get much more I/O's (to be exactly 72/18 = 4 times)

You better make a backup and restore it to the new disks.
Uwe Zessin
Honored Contributor

Re: Different disk types in a RAID 1+0 array?

Mike,
you are correct. If this is a HSG80 he will loose access to the data, because it is suddenly mixed with data from the second mirrorset due to the striping of both mirrorsets. You can also mix mirrorsets with different sizes, but again, the capacity is calculated from the smallest mirrorset.

He can retain all data if this is a SmartArray controller, because it works a little bit like the EVA. But not much - the money must be somewhere ;-)


Arjo,
I am afraid that is not correct - the SA5300 does not exactly work like the EVA. Here is what the user guide (135606-006, March 2003) says:

* For the most efficient use of drive space, do not mix drives of different capacity within the same array. The configuration utility treats all physical drives in an array as if they have the same capacity as the smallest drive in the array. The excess capacity of any larger drives is wasted because it is unavailable for data storage.
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Leif Halvarsson_2
Honored Contributor

Re: Different disk types in a RAID 1+0 array?

Hi,
Of course it is not possible to stripe 18 and 72GB disks in the same RAID set without wasting a lot of disk space.
What is possible (if your RAID controller supports multiple RAID sets) is to create two separate RAID 1+0 sets one 18GB and one 72GB. After that you have 90GB disk space, but in two RAID sets and two LUNs.
Then you can "merge" the two LUNs on OS level into one large volume.