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VSICM55 - Slide 07-21 - Creating an RDM



1. Wrong:

When describing physical compatibility mode RDMs (pRDM) limitations, slide notes state:

LUNs configured for pass-through mode can be a maximum of 62TB in size.
1. Correct:

The actual maximum size of a pRDM is 64TB.

The maximum size mentioned in slide notes - 62TB - is instead the new maximum configurable size in vSphere 5.5 for virtual compatibility mode RDMs (vRDM).

1. Source:

Configuration Maximums for VMware vSphere 5.5.



2. Room for improvement:

In the bulleted list on the slide notes, when referring to the compatibility modes supported by RDM, you can read that:

Physical compatibility (pass-through) mode [...] a LUN configured for physical compatibility cannot be cloned, made into a template, or migrated if the migration involves copying the disk.
Virtual compatibility mode [...] When you clone the disk, make a template out of it, or migrate it (if the migration involves copying the disk), the contents of the LUN are copied to a virtual disk (.vmdk) file.
2. Correct:

The above statements are either incomplete or incorrect. The described Storage vMotion behavior was a limitation when migrating RDMs in vSphere 4.x, yet in vSphere 5.x it has been improved.
Cormac Hogan - a senior technical marketing architect within the Cloud Infrastructure Product Marketing group at VMware - has further clarified RDM behavior during Storage vMotion operations.

  • 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 behavior 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).
2. Source:

VMware vSphere Blog.


Last modified onWednesday, 15 January 2014 19:04
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