RAW DEVICE MAPPINGs

[joeflint]

Hi,

I'm looking for guidence and advice on RDMs and how to go about configuring them.

Basically, we have a number of VMs and these require connectivity to one or more LUNs. Are RDMs required? If so, how does one connected a VM to the RDM?

Thanks

[vmroyale]

Hello.

Basically, we have a number of VMs and these require connectivity to one or more LUNs. Are RDMs required?

Yes, if you need direct LUN access then you can use RDMs to accomplish this.

If so, how does one connected a VM to the RDM?

Basically you create the LUN and present it to your host(s) the same way you would a VMFS volume. Then you add it to a virtual machine, just like you would add a VMDK. You just choose the RDM option instead. The full information on RDMs and connecting to them can be found in the ESX Configuration Guide.

Good Luck!

[Cyberfed27 · Accepted solution]

Basically an RDM is a formatted LUN volume (windows, linux etc..) that already exists.

If you want to move that LUN away from a physical server and apply it to say a virtual machine while preserving the data on that LUN, then that's when a RDM comes into play.

You will need to apply the appropriate zoning or mapping on your SAN so that the physical VMware host can access the LUN (essentially removing its acces from a physical server and moving it to the vmHOST). The same way you present a lun to your vmHOSTs on your SAN.

From there you can add the disk to any VM as you would add a new disk to a VM in its properties, the difference is you select the RDM option instead.

This works great, remember you have a 2TB size limit on RDM's ( I think its 2TB 'officially')

Save yourself some headache and make sure the datastore where the VM lives has a block size large enough to support the RDM you are attaching.

This will be important for cloning and snapshot purposes.

That's all there is to it.

I recently moved a 1.5TB LUN attached to a physical host to a VM via RDM when the physical host died. I was able to preserver the entire volume and had no issues.

[joeflint]

Thanks for the explanation - a VM is able to connect to more than one LUN?

[Cyberfed27]

Absolutely. Assuming permissions on the LUNS are set correctly on the SAN.

What you don't want to do is share LUN access to two servers at the same time (unless in a clustered setup where there is a watchdog).

For example you dont' want server A and Server B to both have access to a windows LUN. That's bad times.

[jlitnyc]

I see that you were able to move a LUN from a physical to tha virtual host successfully.  Have you ever tried to do the opposite?

I have a VM with an attached RDM that needs to be moved to a physical host.

Has anyone tried this?

[sa2057]

Hi Jlitnyc,

"I have a VM with an attached RDM that needs to be moved to a physical host.

Has anyone tried this?"

We tried this and it works but in order to acomplish this you have to follow few steps..

I assume it is windows VM..

Take a complete backup of that specific Lun

Set the drive offline from windows disk management

Remove the lun from the vm...

From the SAN end ---Remove the lun number from the ESXi host so that they can't see the lun any more.

                               Add the lun to physical machine

Assuming the physical server iscsi initiator is already configured.You can configure the rest on the physical machine

Thanks

SA