When working with virtual machines, it is often advantageous to over allocate and under utilize resources. When it comes to virtual hard disks, this is even more common place. On low use or low demand servers, I always use thin provisioning. This saves disk space by only using physical disk space when the guest actually uses the virtual disk. But what about those disks that were created using the thick option, or brought over as thick automatically during a P2V conversion? Time to convert your thick virtual disk to thin.
As always, I recommend backing up all of your data and knowing what you are doing. Test this in a non-production environment.
Converting disks from thick to thin is actually quite easy and can be accomplished using these steps:
1. Log into your ESX host using SSH and cd into the VM directory that contains your virtual disk.
2. Shutdown the VM so that we can get exlusive access to the virtual disk.
3. Run vmkfstools -i yourthickdisk.vmdk -d thin yourthindisk.vmdk
4. Edit the settings for your VM and remove the existing drive. Add a new hard drive and choose the existing drive option.
5. Boot the VM and enjoy.
Note: Dont forget to go back to ESX server and remove the old .vmdk and -flat.vmdk files once you are sure that your VM is operating normally off the thin disk.
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Here is an script that does that and more automatically, for as many VMs as you want.
It can also give you a summary of storage savings.