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RAID Groups

Hi All

I'm a bit of a newbie to V-Series.

Can we create RAID groups of only 1 Lun each and then use all the RAID groups to create an aggregate? The reason I ask is that we have luns of varying sizes ranging from 100GB to 400GB in size on an existing storage array (all in separate array raid groups) and I am looking at creating a large aggregate of around 2TB without having to re-org the whole of the storage array.

Am I better creating multiple RAID groups of same size disks?

What are the down sides of doing this?

TIA

JP

Re: RAID Groups

Hi,

Have a look at this doc:

http://www.netapp.com/us/library/technical-reports/tr-3461.html

Three things from there:

- Data ONTAP uses RAID 0 to stripe across the array LUNs, which splits data evenly across two or more array LUNs

- The best practice is not to span the aggregate across multiple arrays.

- Each V-Series controller has a maximum capacity and a maximum number of LUNs it can support. Using small-sized LUNs will result in a V-Series system reaching its LUN count limit before reaching its capacity limit

So the answer to your actual question is yes, you can use existing LUNs (providing they are of the same type, say FC), but be wary of the 3rd bullet point above.

Regards,
Radek

Re: RAID Groups

Thanks Radek I had read that and the planning and setup guides for V-Series and although they are helpful they do seem to assume that you are starting with a greenfield storage array.

Also the planning guide does mention using single luns in an aggregate but then the Best Practice says create RAID Groups with 2 or more luns and then the setup guide says when you create an aggregate you must use at least 2 Luns. See beow.

ndisks[@disk-size]
ndisks is the number of array LUNs to use. The value must be
at least 2.

It is maddeningly confusing to say the least. I get the impression that no-one really knows.

Cheers

John

Re: RAID Groups

Hi John,

I am not quite sure whether you can, or cannot create an aggregate using only 1 LUN (the author of the TR doc is a regular here, so he will certainly peek into our thread at some point & answer that though)

In my opinion the important question is: why would you like to do that?

If you have a bunch of LUNs why do not group them together into single aggregate to get better performance? You can carve as many volumes (well, up to the limit) as you like from the single aggregate & use them accordingly for whatever (diffrent) purposes.

Does it make sense?

Regards,
Radek

Re: RAID Groups

Hi John,

     We advise to keep LUNs in the same aggr roughly the same size.  What can happen over time if you have, for instance, 10x 100g LUNs and one 400g LUN in a single aggr, is that as the 100g LUNs fill up, there will still be 300g of free space, but it will all be in that single LUN.  The result will be what we kindly refer to as "uneven" performance.  Once those smaller LUNs fill up, they are no longer handling write IOPS.  It's certainly possible, and there are people doing it.  But it can (and probably will) cause performance problems.

     You are correct that most of the planning guides presume dedicated array RAID Groups.  Our best practice is to not share RG's or ports with other array hosts.  Again, the concern is "uneven" performance.

You should figure out, for each array RG, how much space is available to us.  Based on that, we can figure out the best way to carve it up.   Feel free to follow up with me over email with some more details, and I could offer you some more specific advice.

Thanks!

Daniel Isaacs
Technical Marketing Engineer  - VBU
V-Series

NetApp
Daniel.Isaacs@netapp.com
www.netapp.com