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Cormac_Hogan
Posted by Cormac Hogan
Technical Marketing Manager (Storage)

On a number of occasions recently, I had to investigate what happened to a Raw Device Mapping (RDM) when:

  • The VM to which the RDM was attached was Storage vMotion'ed (VM Powered On)

 

  • The VM to which the RDM was attached was Cold Migrated (VM Powered Off)

 

Some of you may even have been following along the comments in some of my previous postings. Well, this is what I observed, testing with both pRDMs and vRDMs.

 

VM with Physical (Pass-Thru) RDMs (Powered On – Storage vMotion):

  • If I try to change the format to thin or thick, then no Storage vMotion allowed.
  • If I chose not to do any conversion, only the pRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN.

 

VM with Virtual (non Pass-Thru) RDMs (Power On – Storage vMotion):

  • On a migrate, if I chose to covert the format in the advanced view, the vRDM is converted to a VMDK on the destination VMFS datastore.
  • If I chose not to do any conversion, only the vRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN (same behaviour as pRDM)

 

VM with Physical (Pass-Thru) RDMs (Powered Off – Cold Migration):

  • On a migrate, if I chose to change the format (via the advanced view), the pRDM is converted to a VMDK on the destination VMFS datastore.
  • If I chose not to do any conversion, only the pRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN

 

VM with Virtual (non Pass-Thru) RDMs (Power Off – Cold Migration):

  • On a migrate, if I chose to covert the format in the advanced view, the vRDM is converted to a VMDK on the destination VMFS datastore.
  • If I chose not to do any conversion, only the vRDM mapping file is moved from the source VMFS datastore to the destination VMFS datastore – the data stays on the original LUN (same behaviour as pRDM).

 

As you can see, there are 3 occasions when an RDM could be converted to a VMDK. Perhaps the most surprising is the fact that a pRDM could be converted to a VMDK, when a cold migration of the VM is attempted, and the format is changed.

I've since asked our engineering team to put a warning into the migration wizard in vSphere to highlight that this is what's going to happen. Right now, you don't get any warning about this.

I wanted to finish this post with a question to the community. How useful would you find an RDM -> RDM migration tool, i.e. the ability to move data from one LUN to another LUN via the vSphere migration wizard? Please leave me a note in the comments if you think you would use this?

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