Tag Archives: RDM

Moving from an RDM to a VVol

Migrating VMDKs or virtual mode RDMs to VVols is easy: Storage vMotion. No downtime, no pre-creating of volumes. Simple and fast. But physical mode RDMs are a bit different.

As we all begrudgingly admit there are still more than a few Raw Device Mappings out there in VMware environments. Two primary use cases:

  • Microsoft Clustering. Virtual disks can only be used for Failover Clustering if all of the VMs are on the same ESXi hosts which feels a bit like defeating the purpose. So most opt for RDMs so they can split the VMs up.
  • Physical to virtual. Sharing copies of data between physical and virtual or some other hypervisor is the most common reason I see these days. Mostly around database dev/test scenarios. The concept of a VMDK can keep your data from being easily shared, so RDMs provide a workaround.

Continue reading Moving from an RDM to a VVol

Comparing VVols to VMDKs and RDMs

I have been talking a lot about Virtual Volumes (VVols) lately with customers and when I describe what they are a frequent response is “oh so basically RDMs then?”. ..

…ugh sorry I just threw up in my mouth a bit…

The answer to that is an unequivocal “no” of course, but the question deserves a thorough response.

So first let’s look at how they are the same, then let’s look at their differences. And not just how they compare to RDMs, but also VMDKs as you traditionally know them.

Continue reading Comparing VVols to VMDKs and RDMs

Migrating a Raw Device Mapping with Federated Live Migration

Migrating a virtual machine that uses 100% virtual disks is a simple task due to VMware Storage vMotion but migrating a VM that uses Raw Device Mappings from one array to another is somewhat trickier. There are options to convert a RDM into a virtual disk but that might not be feasible for applications that still require RDMs to be supported. Other options are host based mechanisms/in-guest mechanisms to copy the data from one device to another. That option sometimes can be complex and requires special drivers or even possibly downtime to fully finish the transition. To solve this issue for physical hosts, EMC introduced for the Symmetrix a feature called Federated Live Migration.

Federated Live Migration (FLM) allows the migration of a device from one Symmetrix array to another Symmetrix array without any downtime to the host and also does not affect the SCSI inquiry information of the original source device. Therefore, even though the device is now residing on a completely different Symmetrix array the host is none the wiser. FLM leverages Open Replicator functionality to migrate the data so it has some SAN requirements–the source array must be zoned to the target array. A FLM setup looks like the image below:

flmSAN Continue reading Migrating a Raw Device Mapping with Federated Live Migration