One raid group vs multiple raid groups for a file system

Question

We have a 4+1 Raid 5 group.  Only one host (AIX) will be using that Raid group.  We need to create four LUNs for a single striped AIX file system.  Two LUNs on SPA and two LUNs on SPB.  Is it efficient to have all four LUNs on the same raid group (same set of disks)?  Or would we be better off creating four different raid groups with one LUN from each Raid group. It seems like having all four LUNs on the same Raid group may create a disk thrashing scenario.  That is because it is striped on the array and then we need to stripe it at the OS.  If we don't stripe it at the OS then AIX will completely fill up the first LUN before writing anything to the second LUN etc.  We want AIX to fill up all  4 LUNs at the same time.

driskollt1 · Accepted Answer

Striping within the same RAID Group is never a good idea.

You would be better off creating 1 LUN within that raid group than 2 LUNs in the same RAID group and striping them.

Balancing between SPs is good when you have a lot of spindles and RAID Groups involved. I wouldn't worry about it for 1 - 5 disk RAID Group. Still you should NEVER stripe the same data within the same RAID Group. It will hurt your perfromance, rather than help it.

If you really want to have at least 2 LUNs striped, then 2 RAID Groups with be what you should use.

If you have another server with some striped LUNs you could... (make sure both servers wouldn't be super busy at the same time)

Create 2 RAID-5 RAID Groups - let's call them RG0 and RG1

Server-1 - Create 2 LUNs - let's call them LUN0 and LUN1

LUN0 - SPA - RG0 - Primary (First) LUN for the stripe

LUN1 - SPB - RG1

Server-2 - Create 2 LUNs - let's call them LUN3 and LUN4

LUN3 - SPB - RG0

LUN4 - SPA - RG1 - Primary (First) LUN for the stripe

kelleg · Answer

Using four LUNs from the same raid group and then stripping - either by the filesystem or by using metaLUNs on the array will cause disk contention.

If you need to stripe then you should create four raid groups and use one LUN from each RG.

This is discussed in the White Paper about metaLUNs - in the Documents section on the forum:

White Paper EMC CLARiiON MetaLUNs - A Detailed Review.pdf

glen

driskollt1 · Answer

Is there a reason you need to create 4 LUNs for one file system?

Since you're only spanning it across 5 disks in a RAID-5 RG, I'd assume the I/O isn't going to be that intense. If that's the case, a single LUN in the RG with one SP owner would probably give you better performance than creating multiple LUNs in the same RG and striping them. I've seen cross plaids that are only moderately busy drive queue lengths (which also translates into increased response time) through the roof.

4 LUNs from different RAID groups would be a better option if you need 4 LUNs. If you don't need a lot of capacity, maybe 4 RAID-10 RGs with 2 disk each would suffice? That would only be 8 disks and give the same capactiy as the 4+1.

brad12341 · Answer

Actually after re-thinking I could get by with fewer LUNs. But would I be better off with 2 LUNs in the same raid group rather than one? If I have only one LUN, I will not be able to balance the load between SPA and SPB.

The best solution would be to get more disk and have more raid groups. Just don't know that I can get more disk right now.

kelleg · Answer

If this is the constraint that you must live with, then two LUNs in the same raid group - do NOT stripe on the array - assign one to SPA and one to SPB then stripe on the host - you will lose some performance this way, but it's better than stripping on the array and then stripping on the host - use one or the other but not both.

glen

CLARiiON

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