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Large Database on virtualized SQL server 2008 (vmware)

Posted on 2010-08-25
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Industry experts seem to say that large databases (>500-800GB) should be placed on Raw Luns. Why? What happens if you just use a VMDK?
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Question by:Johnjaysmith
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by:mcv22
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Using virtualization of any kind has a performance overhead. If performance is not a concern, virtual disks should suffice.
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by:Johnjaysmith
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So Raw Luns are not virtualized? What is their structure as opposed to a vmdk?
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by:coolsport00
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No...RDMs are pointers to LUNs... not actual virtual disks.

~coolsporr00
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by:bgoering
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If you have no reason to cluster your sql server with some other physcial or virtual host, there is not any reason to use rdms. However, when allocating LUNs for your vmdk files normal database considerations apply. For example I will allocate a RAID 10 LUN to my ESX servers, and place database log datasets there. I use RAID 5 for about everything else.

As of VMware 3.5 there is not much difference in performance between rdms and vmdk files.

Good Luck
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by:Johnjaysmith
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Your question implied to me that clustering a sql server requires or does better with rdms (raw lun)?
Can you create a raid array with an rdms disk format?
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coolsport00 earned 200 total points
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Again, an RDM is solely a "pointer file", nothing more. It "points" to LUN disks. So, to answer your question...yes. Your LUNs can be whatever RAID your SAN supports. RDM provides 'direct access' to your SAN, whereas a virtual disk (VMDK) is storage on a VMFS volume (datastore), which is more than likely a LUN on your SAN (but obviously doesn't have to be).

Hope that helps.
~coolsport00
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by:bgoering
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Yes, if you have a requirement to build a cluster that crosses physical host boundaries, could be virtual to physical, or virtual to virtual if each instance is on a seperate esx host server, you will need to use rdms for any luns to be shared between the servers. The local OS disks can be vmdks.

Creating the lun would be a function of your storage, create a lun like you would for anything in whatever raid format you want. When you present your lun to ESX, instead of adding it as a datastore to ESX, create the rdm link in the database  server configuration.
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