Re: RAC and ASM disk layout

Hi Helmut,

"all LUNs will end up on the same physical devices anyway. So it might be harmful having ASM to try and balance I/O across several LUNs which all reside on the same physical devices" This is true, and if you do that you will get a lot of disk contention. As of RAID-5, the general rule of thumb is to deploy RAID-5 where cost of storage is critical, performance is not a primary goal, and applications with primary read operations like DWH. Since ASM is based on the SAME technology and is LVM itself, using LVM with ASM is not a good idea since it would complete a duplicating of functionality, overhead and complexity.
ASM striping with a hardware RAID-0 is OK if configured correctly. In that case use 1Mb stripe size. The smaller than 1Mb stripe size is inefficient => breaks down the 1Mb I/O into multiple smaller I/Os and because a physical disk constraints.

If you have a 5 disks per EMC available, I would suggest, do not create one LUN on this five disk, but rather each disk to be one LUN, and group this 5 disk per EMC in one failure group...and let the ASM do the striping and mirroring with another box (second failure group).

Even if a lot a people think that ASM is a "black box", it is a great tool...and if you know the way how it works, and configure it correctly, you can make your life much easier.

Mit freundlichen Grüßen,

Goran

Subject: RAC and ASM disk layout
Date: Fri, 9 Jun 2006 12:41:43 +0200
From: "Daiminger, Helmut" <HELMUT.DAIMINGER_at_wwk.de>

Hi!

We are new to RAC and are discovering serveral uncertainties regarding the best disk layout to use for ASM. We are running two EMC Clariion systems (under RAID-5) and want ASM to write to both Clariions simultaneously (i.e. use normal redundancy). ASM will balance the load across the two failover groups (i.e. two LUNs) which reside in two EMC Clariion arrays.

The question is, whether it is better to use a single LUN with 100 GB or have a number of smaller LUNs (totalling 100 GB). Since we are using a single raid group (i.e. 5 disks) for all database storage, all LUNs will end up on the same physical devices anyway. So it might be harmful having ASM to try and balance I/O across several LUNs which all reside on the same physical devices.

This is 10.2 on HP-UX 11.23.

Thanks,
Helmut

--
http://www.freelists.org/webpage/oracle-l