RE: ASM - number of LUNS rule of thumb

From: Luca Canali <>
Date: Thu, 12 Aug 2010 09:34:45 +0200
Message-ID: <>

Hi George,

LUNs to be used for ASM cannot be larger than 2TB, a technology limitation, so this will also set a minimum number for the LUNs you will need, depending on the size of your DB.

Our experience is with using ASM and normal redundancy on low-cost storage, in that case one would export 1 LUN per disk (JBOD config) so we have systems with more than 100 LUNs and having a large number of LUNs does not seem to be a problem for Oracle or Linux. When using external (RAID) redundancy I imagine one prefers to minimize the number of exported LUNs for simplicity as most details are taken care at the storage level. Anyway details will vary depending on how the storage is set up, so it's hard to give a general recommendation, I believe.

One large DATA and FRA diskgroups are easier to manage, and also ASM is stable and mature enough that it reasonably allows for such an architecture. Also if you have a single host (no clustering?) I would say you have to think globally about your HA requirements and in that picture consider the risk of an ASM/storage-related issue blocking all your DB activities on the given node. Having said that the day you will have a corruption on one diskgroup, if ever, you'll probably very much prefer to have gone for the solution of having multiple diskgroups :)


-----Original Message-----
From: [] On Behalf Of Johnson, George Sent: 11 August 2010 17:13
Subject: ASM - number of LUNS rule of thumb

        I wanted to ask the list some advice on a particular ASM "rule-of-thumb" and advice on diskgroups per host.

        We have recently decided upon using ASM, yes I know late to the party, we have studied quite a few white papers and they often suggest that you start somewhere between 4 to 8 luns maximum per diskgroup as a rule-of-thumb. I don't like rules-of-thumb, they suggest that no-one has any idea and it's a best guess! What do others have as a starting point basis for the number of raw luns under each diskgroup and why that number? I appreciate there are many more factors involved and we are slowly working through lots of these, but I am trying to get some facts based on experience, rather than simply taking documented suggestions as gospel truth.

        Additionally, my colleague and I are arguing over the merits of the number of diskgroups per host. Say you have 4 instances on a host, does each instance have at least two diskgroup each ( DATA+FRA for example) , or do you simply make big diskgroup and throw all the instances into it? The "all-in-one" options suggest 'eggs-in-one-basket to me though.

        Any advice from your vast collective experience on these two points, would be greatly appreciated.

	George Johnson

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i0zX+n{+i^ Received on Thu Aug 12 2010 - 02:34:45 CDT

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