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Recently, I was tasked with migrating our entire VMware implementation to a new storage array.  Essentially, we are replacing our current SAN with one that is claimed to be bigger, better, and faster.

That sounds great, but how do you move 35 TB of data belonging to 800 VM’s with a minimal impact?  Storage VMotion may be what you’re looking for.

Storage VMotion can be a really useful tool.  Essentially, it can let you move a VM from one datastore to another while the VM is powered on, much like how a standard VMotion can let you live-migrate between hosts.

There are several considerations to keep in mind when preparing for a Storage VMotion.  A more complete list can be found here, but the more common ones we ran into were:

     –  Virtual machines with snapshots cannot be migrated using Storage VMotion.
     –  Virtual machine disks must be in persistent mode or be raw device maps.

The first one should be fairly obvious: you must delete any snapshots before performing an online Storage VMotion.

The second has to do with the disk “mode”.  You can research more about the different mode options but essentially the virtual disk must be able to be snapshotted (i.e. not “independent”) and the changes must be persistent (i.e. not discarded when the VM is powered off or a snapshot is deleted).  This is a requirement as the snapshot functionality is used by the Storage VMotion process.  If you have disks that may be marked as independent, you can plan to power down the machine and change this setting for the duration of the migration, or just migrate it while its off.

There are few different ways to perform a Storage VMotion which I’ll describe in a future post.