Both Hyper-V and VMware allow virtualization administrators to perform storage migrations -- moving all or a portion of a virtual machine's files from one storage location to another. Even though both vendors try to make this an easy move, no admin should take the process lightly.
Is a storage migration really necessary?
If your current storage is running out of space or not delivering sufficient IOPS, you may assume it is time to upgrade and migrate. Sometimes, however, you might be able to add a physical disk to your existing storage to increase capacity and performance, without having to take on a full-blown storage migration.
The next decision you will have to make regarding a storage migration is which components you plan to move. In most cases, storage migrations involve moving all files that comprise a virtual machine (VM).
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Occasionally, volume-capacity limitations force administrators to move specific VM components rather than entire VMs. For example, if a VM has an excessively large virtual hard disk file, an administrator might need to move just that one virtual hard disk as opposed to the entire VM. Similarly, an administrator might make the decision to move VM snapshots to a dedicated volume, and leave all other VM files in place.
Next, choose a new storage location. Obviously, the new storage hardware must have sufficient capacity and performance for your VMs. Another important factor to consider is whether the storage location will be accessible to your virtualization host cluster.
If you decide to use a Server Message Block (SMB) volume as opposed to a Cluster Shared Volume, as Windows Server 2012 Hyper-V permits, then you must make sure that the server hosting the volume is running a supported version of the SMB protocol.
How will a storage migration affect VM performance and backups?
Any time that you move VMs or VM components from one storage location to another, there is a very real chance that the move could affect your backups. Often times, an administrator simply needs to check backup settings to make sure that the backup software uses the new storage location.
Sometimes, however, permission issues can occur. For example, your backup software might not have permission to access the volume that will store the VM components after the storage migration. As such, it is important to evaluate the impact that the storage migration will have on your backup, and address any issues before actually performing the storage migration.
As you plan for the storage migration, you should also take into account how the migration process will affect any running VMs. Prior to the release of Windows Server 2012, Microsoft only supported what it called Quick Storage Migrations.
A Quick Storage Migration creates a brief period during the migration process that VMs are inaccessible. Windows Server 2012 Hyper-V still supports Quick Storage Migrations, but it also supports live storage migrations, which allow VMs to remain accessible throughout the migration process. VMware offers a similar process to prevent VM downtime.
In spite of the fact that mechanisms exist to ensure VMs remain accessible during a storage migration, the accessibility of a VM during the migration process ultimately boils down to the capacity of the underlying hardware. The storage migration process uses a lot of the server's disk resources. If the server is unable to keep up with the demand for disk I/O, then VM accessibility will suffer -- even if a mechanism exists to keep VMs accessible during the migration.
As you can see, there is a lot of planning that goes into a storage migration. One often-overlooked aspect of the planning process is ensuring you have a good backup in place before you begin. That way, you will have a way to revert data back to its previous state if something should go wrong during the storage migration process.