Simple definition
VMDK is a virtual-disk format commonly used in VMware environments. Descriptor and associated data files together represent storage exposed to a virtual machine.
Technical operation
Depending on the product and storage backend, VMDKs can use thin or thick provisioning. One logical disk may consist of several files. Provisioning affects available capacity and the risk of allocating more virtual space than physical storage.
VMware snapshots introduce delta disks that depend on a parent disk. Long snapshot chains increase consolidation complexity and may reduce performance. Removing snapshots needs free space and is not a replacement for backup.
Practical example
For example, a VM presents a 500 GB disk while a thin-provisioned VMDK consumes only 80 GB on the datastore. Multiple growing VMs can still exhaust that shared datastore.
Best practices and limitations
Monitor free capacity and datastore latency, check consolidation operations, regularly test backups, and verify compatibility before converting a VMDK to another hypervisor format.