We recently had need to evacuate one of our busy SAN arrays. For the most part, this was easy enough. Thanks to the storage vMotion (SvMotion) capability, it was easy enough to move the virtual machines without much difficulty using the normal migrate processes while the vm’s were up and running.
The trouble comes with Raw Device Mappings (RDM’s). We have a general policy that when a large disk is needed, it is added as an RDM, as opposed to taking up an entire large datastore for this purpose. This has worked very well for us in the past, but in this case, it caused a bit of an issue. First, we needed to migrate this RDM, and since our SAN-based replication/migration tools did not cross vendor boundaries, this was not as simple as pushing a button.
We needed to do the change with as little disruption as possible, and to avoid downtime as a general rule. Now, going back to the SvMotion, if you leave the defaults in tact, the RDM is moved to a new datastore as an RDM. You can, however, change this behavior by taking a couple extra steps.
| First, select migrate as you would normally, and select Change Datastore. |
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| Next, select your destination vm location, cluster/resource pool, etc. |
| When you come to the select datastore screen, click Advanced Button. |
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| In this screen, you can individually select where you would like to place all of your disks, as well as change the location of the vm configuration file (working directory). That is the location of the vm swap file, and any snapshots that are taken |
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| The next screen is the most important. It is where you tell vSphere/ESX that your intention is to actually convert the RDM to a vmdk. Change the appropriate radio button for whether you want your new vmdk to be either thin or thick provisioned |
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| Verify in the summary page that what you picked the correct datastores, and just wait for the task to complete. |
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| There is one caveat to this conversion. Before ESX 5, only virtual mode RDM’s could be converted to thick/thin vmdk files. I happen to still be running 4.1, so I had to change the way RDM’s were presented to a few vm’s before I could SvMotion the data off. |
-KjB
