VMware Virtualization – RDM to VMDK Migration

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House of Brick Principal Architect

A fun customer of ours asked me today if you could convert an RDM directly into a VMDK. They had a SQL Server instance residing on a Fusion-io SSD card through a pass-through Raw Device Map (RDM) on a VMware ESXi 5.0 host. They wanted to migrate the SQL Server to the SSD-based area on their SAN so that the VM could benefit from VMware HA between ESXi hosts in the vSphere cluster rather than pinned to a single compute node.

At first, I did not think this was directly possible. However, we found a VMware knowledge base article (1005241) that says you can do it. The methodology behind it makes perfect sense.

The method behind the migration depends on your current RDM configuration. Two kinds of RDMs exist – RDM-Physical (RDM-P) and RDM-Virtual (RDM-V). As my wizard-like colleague Jim Hannan (@HoBHannan) noted in his recent blog post ‘VMFS vs. RDM’, RDM-V specifies full virtualization interception of the SCSI commands to the storage device, and snapshots can be taken. With RDM-P, the SCSI commands are directly passed-through to the storage. VMware-level operations are very limited, but full SAN tooling exists.

Raw Device Map – Virtual Options

If your storage pointers are configured in the RDM-V manner, your options are better than RDM-P. You have to methods – cold migration and Storage vMotion (if the licensing level allows for it).

First, if you have the option for a period of downtime, the simplest method is to gracefully shut down the VM and perform a cold migration. You don’t even have to migrate between hosts. The migration process will create a new virtual disk and will clone the RDM into the virtual disk.

The original mapped LUN is left alone after the migration completes, and can be removed as necessary to reclaim space.

You can also use a Storage vMotion operation to perform the same migration task.

If you are moving SQL Server or Oracle data from RDM to VMDK, please use the advanced tab in the migration wizard to specify ‘Thick Provisioned Eager Zeroed’ as the virtual disk format. It ensures that the entire space required for the database objects is contiguous on the storage, yielding optimal storage performance.

Raw Device Map – Physical Options

If you have the RDM configured as RDM-P, you cannot migrate the storage using a Storage vMotion. Cold migrations are still fully supported.

The end result is that you can successfully (and simply, to boot!) migrate from RDMs to VMDKs using fully supported methods.

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