Re: ASM and RAID

From: Michael Austin <maustin_at_firstdbasource.com>
Date: Mon, 19 Jan 2009 21:09:06 -0600
Message-ID: <qfbdl.11683$as4.5742_at_nlpi069.nbdc.sbc.com>



Robert Klemme wrote:
> On 18.12.2008 17:32, Michael Austin wrote:
>> 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).

>
> That's an interesting bit of information. How is the ASM able to
> replace a clustered file system? Does it provide only the meta data
> layer which controls concurrent access to files?

Sorry taking so long to get back to this thread...

In order to have a "clustered file system" one of the prerequisites is to have some method of I/O fencing - a mechanism to prevent both servers from writing the same data block at the same time. This is what Cache Fusion which includes the Distributed Lock Manager (DLM) is for - to allow concurrent writes to raw devices from multiple servers in a RAC cluster.

If you ever worked in an OpenVMS environment - this is how they made it work... The DLM is the most important piece of this whole thing. Without it, you would have chaos.

>

>> There is a book called Automatic Storage Management - practical 
>> under-the-hood ??? that is very good at the mechanisms within ASM and 
>> RDBMS...

>
> I am assuming you mean this one: http://www.amazon.com/dp/0071496076

Yes.

>
> Kind regards
>
> robert
Received on Mon Jan 19 2009 - 21:09:06 CST

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