Re: ASM and RAID

From: Michael Austin <>
Date: Thu, 18 Dec 2008 10:32:34 -0600
Message-ID: <SUu2l.8351$>

DA Morgan wrote:
> robert wrote:

>> Michael Austin wrote:
>>>  How, in your
>>> testing and research, does performance either improve or suffer with 
>>> the addition of ASM.  The database is scattered across 6 separate 
>>> disk groups with logfiles etc in their own disk group.
>> I've no numbers to contribute (though I'm also very interested in the 
>> collective experience of this group on this).
>> But I do have a question: how much would we reasonably *expect* 
>> performance to be impacted one way or the other by ASM, when we are on 
>> a high performance SAN?

> On a high end SAN you are reading and writing to a cache not to disk so
> the number of spindles becomes less important. This is part of what
> makes NetApp's RAID4 different from RAID4 from other vendors.

Not sure, but doesn't this statement make my point from earlier RAID5 and SAN discussions :)

> ASM seems to prove either a neutral or slightly positive impact (a
> few percent) on performance if compared to raw disk with no LVM. If
> comparing with other LVMs what we see is a substantial drop in CPU
> utilization. Most LVMs are cpu pigs: ASM is not.

ASM also does not do the actual reading/writing - the RDBMS does. In other words - ASM does not proxy the I/O for the RDBMS - RDBMS writes directly to the data files. ASM just tells the RDBMS what the extent map is and only at file open time... which is why you need additional shared_pool (1MB for every 100GB of file space).

There is a book called Automatic Storage Management - practical under-the-hood ??? that is very good at the mechanisms within ASM and RDBMS... Received on Thu Dec 18 2008 - 10:32:34 CST

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