Adding LUN's or datastores in VMware

Garry White
Garry White used Ask the Experts™
We are using VMware 5.0 and I have been tasked with trying to add multiple LUN's from our EMC San to the one Redhat 6.5 VM. I have not found any documentation about using multiple LUN's from a SAN to one VM. Is this even possible? Is it possible to add multiple datastores to one VM. Usually we have just one LUN for VM or one large LUN with Multiple VM's, but not the other away around.
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Yes Gary,

That's possible.

What you are trying to do is to present multiple LUN as Raw Disk Mapping (RDM) to the Linux VM.

Virtual compatibility mode provides vSphere snapshots of this virtual disk.
Physical compatibility mode allows the VM to pass SCSI commands direct to the storage system LUN. This allows it to leverage SAN specific features such as interaction with the SANs own snapshot function

Assuming you have already assigned the LUN to the ESXi server group, here are the steps from the vSphere console Windows Client:

1. The first step to adding a RDM to a virtual machine is to assign an unused LUN to your ESXi Servers. This will vary depending on the type of SAN in use (consult the SAN vendors documentation on how to do this).

2. For the new LUN to become available, Rescan the HBAs on all your ESXi servers.

3. The virtual machine that the RDM is going to be added to needs to be shut down first, the RDM cannot be added while it is running.

4. On the hardware tab, click "Add" and choose "Hard Disk".

5. For the disk type choose "Raw Device Mapping" (RDM). Click Next.

6. Choose the LUN. Click Next.

7. Choose "Store with virtual machine" or if you want to store the link to the RDM in a specific datastore. Click Next.

8. RDM volumes can be created in either "Physical" or "Virtual" compatibility mode as per above explanation.

10. Confirm settings and choose finish.

11. A new SCSI controller and hard disk is added to the virtual machine configuration. Now boot your VM. Depending on the OS check for new disks and format/mount the disk.
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
Fellow 2018
Expert of the Year 2017

Is it possible to add multiple datastores to one VM.

Depends what you want to do, present a RAW LUN, e.g. RAW Device Mapped direct LUN to SAN to VM, and format it with native file system.

or present multiple LUNs to ESXi, create datastores, and then create virtual machine disks for your VMs, on different datastores.

RDMs/RAW LUNs, are not often used these days, unless special circumstances exist like Clustering.

VMDKs are easier to manage, and perform just as well now!

see this link

also see this

RDM versus VMDK performance

Conclusion: VMFS and RDM have similar performance. Don’t choose RDM for performance.
Mr Tortu(r)eSystem Engineer


just to talk about the vocabulary :
- a LUN is a logical disk unit on a San
- a Datastore is logical disk unit on ESX, it can be a whole LUN, or part of it
- a VMDK is a VM's virtual disk, stored on a datastore ; so again it is a logical disk unit, but the last layer one

So you can't "add datastores" to a VM, you add VMDK or RDM to it.
But you can add multiple VMDK to a VM, and these VMDK can belong to different LUN on your San.

RDM is a way to present a LUN directly to your VM, for the VM to "speak" directly to the San as if you presented a LUN to a physical server, so there is no VMDK in this case.
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization Consultant
Fellow 2018
Expert of the Year 2017

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