You probably may have heard, that with the upcoming release of vSphere 5.1, a new virtual machine disk format will be introduced called called SE Sparse (Space-Efficient). One of it's features is to provide the ability to reclaim unused blocks from within the guestOS. I would highly recommend you check out a recent blog post vSphere 5.1 Storage Enhancements – Part 2: SE Sparse Disks by Cormac Hogan for more details about the new SE Sparse disk format as well as other storage improvements in vSphere 5.1.
As Cormac points out, this new disk format will initially be leveraged by VMware View (in a future release from my understanding), as there are additional integrations required to use this feature than just using the new SE Sparse disk format. Having said that, the SE Sparse disk format is a feature of the vSphere 5.1 platform and with that, you do have the ability to create an SE Sparse disk.
Disclaimer: This is for educational purposes only, this is not officially supported by VMware. Please test this in a development environment before using it on actual systems.
There are two methods in which you can create an SE Sparse disk, directly on the ESXi Shell of an ESXi 5.1 host or remotely connecting to an ESXi 5.1 host.
Option 1 - Using vmkfstools on ESXi Shell
vmkfstools -c 10g -d sesparse WindowsXP.vmdk
Here is a screenshot of new SE Sparse disk descriptor file to prove we have successfully created a new VMDK using the new format:
Option 2 - Using vSphere 5.1 API w/modified remote version of vmkfstools
As mentioned, the SE Sparse disk format is a feature of the vSphere 5.1 platform and as so, you can also leverage the vSphere 5.1 API to create a new VMDK using the virtualDiskManager and specifying the new SeSparseVirtualDiskSpec.
Note: Even though the vSphere API reference mentions the ability to set grain size via grainSizeKb property, I have found that it is not possible and just leaving it blank will automatically default to 1024K (1MB) which might be a system default for now.
You can download the modified version of the remote vmkfstools called vmkfstools-lamw which requires the the installation of vCLI 5.1 or vMA 5.1.
Here is an example of creating the same 10GB VMDK using the new SE Sparse disk format:
./vmkfstools-lamw --server 172.30.0.187 --username root -c 10G -d sesparse "[datastore1] WindowsXP.vmdk"
After you have created your new SE Sparse disk, the next logical step is assign it to a virtual machine. Since this is a new feature in vSphere 5.1, you will need to use the new vSphere Web Client to perform the operation as the legacy vSphere C# Client is not aware of this new disk type. You will also need to ensure that the virtual machine is running the latest ESXi 5.1 compatibility and later (virtual hardware version 9).
Once you have added our newly created disk from the datastore, it should now show up in the vSphere Web Client as Flex-SE for the disk type.