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vmproteau
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Straight VMDK or RDM

I will be virtualizing a client file server. It will be a WIndows 2003 Standard server. I will be adding a 1.5TB SAN LUN as an additional volume.

  • Is there a prefeered or best practice disk configuration for disks of this size?

  • Are there any inherent issues with VMDK files of this size?

I was thinking I would do an RDM (virtual compatibility mode). I understand the basic advantages and disadvantages but, I haven't read any recommendations for very large disks.

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esiebert7625
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If you do use a VMDK make sure your block size is the max of 8MB when creating your VMFS volume or you will not be able to create virtual disks over 256GB. With a RDM you can just expand it at the SAN level and then at the OS level. With a VMDK file you will need to add an extent (usually not recommended) or delete your VMFS volume and re-create it. What I do with RDM's is create a VM with 2 drives, one a VMDK file on a VMFS volume, the second drive is my RDM.

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RParker
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There aren't any real problems with running large VM's of this size, RDM or not.

You have seen the benefits and advantages of both, but if you don't need direct SAN compatibility tools or MSCS then there is not real need to use RDM. You can use a VMDK just fine at 1.5 TB

EGRAdmin
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I typically use RDM's when it comes to larger disks.

(Unless I'm presenting one large LUN to a system I will carve that into the needed drives with VMDKs.)

I generally follow these assumptions;

Avoid using extents for larger volumes as they can be broken and you will also want to avoid using striped dynamic volumes on the OS level. They are fine for generic storage however some tools such as MSCS arn't supported under a dynamic or GPT disk.

If it's a single LUN then you're not going to run into any issues other then maybe slower I/O of large files over one pipe to a single drive bay/array processing the request. It's probably negligible. I just spread my I/O into smaller LUN's across my storage loops (IBM ESS) for performance but that means carving up smaller LUN's and formatting striped volumes.

esiebert7625
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I use RDM's for anything over 600GB, just my preference. There is no performance benefit to using RDM's. The below links talk about this...

ESX Server Raw Device Mapping - http://www.vmware.com/pdf/esx25_rawdevicemapping.pdf

VMDK vs. RDM - http://www.vmware.com/community/thread.jspa?messageID=662212

VM Disk Scenario's and Performance - http://www.vmware.com/community/thread.jspa?messageID=661499

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vmproteau
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In my case, it's an HP EVA SAN so, all disks are essentially already striped and I will be creating only one LUN. It sounds like in this configuration, stability and performance appear comparable.

This brings up another question I had. I'll need to test but, what if I need to add 400GB to this volume in the future. Excluding creating a new larger VMDK and migrating the data, I assume my only choice would be to create a new LUN and add an extent.

  • I'll have to try this and see what happens but, through the EVA management server you can increase the LUN on the SAN side. Then within Windows, you can use diskpart to dynamically extend the basic disk so the volume would now be 1.9TB. I'm curious if you could do the same thing with a VM volume that is configured as an RDM. I assume this will not work.

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esiebert7625
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If you do use a VMDK make sure your block size is the max of 8MB when creating your VMFS volume or you will not be able to create virtual disks over 256GB. With a RDM you can just expand it at the SAN level and then at the OS level. With a VMDK file you will need to add an extent (usually not recommended) or delete your VMFS volume and re-create it. What I do with RDM's is create a VM with 2 drives, one a VMDK file on a VMFS volume, the second drive is my RDM.

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Visit my website:

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