Issues with VMDK Image Compression
There are also limitations on what Virtual Disks can be shrunk –
For example, you cannot shrink a virtual disk if:
• You preallocated disk space when you created the disk.
• The virtual machine has any snapshots. To delete a snapshot, choose VM>Snapshot>Snapshot Manager. Select the snapshot and click the Delete button (on Windows) or icon (on the snapshot toolbar on Linux).
• The virtual disk is a linked clone or is the parent of a linked clone.
• The virtual machine contains physical disks.
• The virtual disk is stored on a CD-ROM.
• The virtual disk is an independent disk in non-persistent mode.
You can change the mode of a virtual disk before the virtual machine is powered on. See Excluding Virtual Disks from Snapshots for a discussion of independent disks.