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Recommendations needed on RDM vs VMDK

Posted on 2014-04-25
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Hi,

I currently have an document imaging system software on a Windows 2003 physical server.  I have a 2TB and a 500GB SAN lun attached to this server.  I will upgrade the imaging system to the latest version running 64bit.  I am planning to install the new version on a VM.  I do not have the option to do a P2V so I will start from scratch.  Currently my weekend full backups on this server takes forever due to the fact that it is a lot of data and the data consist of mostly small tiff files.

Now with vSphere 5.5 I was thinking about creating a 3TB VMFS where I can have all the data and use Veeam to backup everything.  However, I am not sure if this is the best route.  Veeam and even VMware recommend the use of VMFS whenever possible but I was hopping to get some "this is what I would do if I were you" from the experts.  

Also, if I go the VMFS/VMDK route, what is the best way to move the data from the lun to the VMDK? Backup/Restore? Copying it over the network?  The data resides on a VNX 5300.

This is not related to the original question, but Veeam is pretty popular so I'm hoping someone might know the answer.  I know Veeam backs up the vmdk files so in theory if I go the VMFS/VMDK route, my backup should be faster as I will be backing up a single big file instead of millions of small tiff files.  Does it work the same way with Veeam if I use RDMs on virtual mode?  Does it still backup a single big file?
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Question by:braishfield
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Andrew Hancock (VMware vExpert / EE MVE) earned 500 total points
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I would always recommend Backup and Restore, or you could use Robocopy to mirror the data across the network both work reliably.

You may also want to have a read of

RDM versus VMDK performance

Conclusion: VMFS and RDM have similar performance. Don’t choose RDM for performance.
Source:http://www.vfrank.org/2011/03/22/performance-rdm-vs-vmfs/

Seelct carefully, whether to use RDMs or VMDKs (alot of advantages of using VMDKs, e.g. backups, vmdk is "portable"

if you are using RDM you will have to rely on SAN Snapshots.

If you Backup a Virtual RDM, it still backs up to a VMDK file in the restore.

see also here:-

http://www.veeam.com/blog/rdms-explained-for-veeam-backup-replication.html
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by:braishfield
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@Andrew, thanks for the quick response.  I am planning to dedicate a datastore for this machine.  If I want to have 3TB of usable space, what size should I give to my lun?
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by:asavener
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The typical use case for deciding to use RDM over VMDK is if you're using Microsoft clustering services, or if there's some other reason why two boxes would need to access the same data.

Another case on vSphere 5.0 and 5.1 was the ability to map LUNs greater than 2 TB.  That use case is no longer valid if you're using 5.5, however.  (And you should really be using 5.5 if you need LUNs greater than 2 TB.)

Otherwise, you're better off with VMDKs in almost every scenario.


For migrating the data, I agree with backup/restore.
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by:Andrew Hancock (VMware vExpert / EE MVE)
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If this is just a datastore for the VMDK, then you will need to work out, how much snapshot space is required, whilst the VM is being backed up.

This rate of change "delta", depends on how active this file server is between backups, we usually work on

Total of LUN Size = 25-35% of VMDK, plus 10%.
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by:braishfield
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@Andrew, let me see if I understood your equation.  
In a 500GB VMDK  the lun would have to be around 688GB when calculating 25% of the VMDK, right.  500 + 125 + 63.  Is the 25-35% for overhead?
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by:Andrew Hancock (VMware vExpert / EE MVE)
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Yes, so we would round up to a 700/800GB LUN.

The overhead is used, because space is only temporary for your snapshot, whilst in backup mode.

so you've got 200-300GB of snapshot space on the LUN, whilst the backup is running, so 200-300GB of data change whilst VM is being backed up.

This can be calculated scientifically, by checking growth of data each day in the vmdk, e.g. block changes.
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by:braishfield
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Thanks for your help.
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by:Andrew Hancock (VMware vExpert / EE MVE)
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That's not an issue.
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