Importing virtual disks of hosted formats in ESXi 5.1

Recently I stumbled over a new issue that really puzzled me. I was trying to import a virtual appliance (the Bitnami ownCloud stack) into my lab ESXi host. This appliance is provided with a virtual disk in 2gbsparse format to make it compatible with VMware Workstation. For using it in ESXi instructions were given to clone the disk into an ESXi supported disk format using vmkfstools like this:

   vmkfstools -i bitnami-owncloud-4.5.5-0-ubuntu-12.04.vmdk owncloud.vmdk

On my ESXi 5.1 box this command promptly failed with a very confusing error message:
  "Failed to open 'bitnami-owncloud-4.5.5-0-ubuntu-12.04.vmdk': The system cannot find the file specified (25)."

Of course the file existed in the current directory, so how could it not be found?!?

After doing some research I finally was able to understand and fix this issue - it is described in VMware KB2036572: In ESXi 5.1 the VMkernel multiextent module is no longer loaded by default, and this module is needed to support virtual disks of hosted formats (like this one). Later I also found a blog post by William Lam about this issue (I must have missed that ...).

The solution is easy: Manually load the multiextent module by running

   vmkload_mod multiextent

prior to the clone command. This worked as expected for me. After cloning the disk you can unload the module again with vmkload_mod -u multiextent.

This is something to keep in mind ...