Thin Provisioning Tips & Tricks :

* Monitoring the datastore space utilization is very critical to prevent running out of space by overcommitment. Leverage vSphere Datastore alarms like "Datastore usage on disks" and "Datastore disk overallocation %" to trigger alerts

* Take virtual disk size, snapshots, swap and other files into consideration while planning for storage space for datastores and not just virtual disk size

* Avoid running guest OS disk defragmentation programs on a virtual disk which is thin provisioned as it might inflate the disk. Using storage vMotion using thin option will defragment the virtual disk

* Use storage vMotion to convert a existing thick virtual disk to thin

* Use thin/virtual provisioning at storage array also if it supports for better efficiency

* Enable alerts on the storage array to monitor the utilization of thin filesystem/LUN

* As deleting data inside the virtual disks cannot claim the space back, storage vMotion using thin option can be used to make the disk thin again.

* VMware FT cannot be used with thin provisioned virtual disks. Enabling FT will make the virtual disks thick

* Avoid using thin virtual disks for applications in guest os which zeroes out the space before actually writing into it

Different virtual disk formats : Thin - vmkernel allocates space only before an I/O is committed. Virtual disk size will be equal to the utilized size and will grow till the user specified size

Zeroed Thick - vmkernel zeroes out the required free space only before an I/O occurs. Virtual disk size will be equal to the user specified size during virtual disk creation

EagerZeroed Thick - vmkernel zeroes all the free space during the creation of virtual disk. Virtual disk size will be equal to the user specified size during virtual disk creation VMs having virtual disk with thick formats can be converted to thin format using storage vMotion in vSphere.

Source: http://www.viveknadar.com/inquisitive/2009/06/vmware-tip-4-thin-provisioning-with-vsphere.html