Understanding VM snapshot size in vSphere

When creating a snapshot, the existing VMDK file is locked and a new VMDK is created, a delta file. If there are multiple VMDKs attached to the VM, separate delta files will be created for each VMDK.

If VMDKs are placed on other LUNs than where the .vmx file is residing, then all delta files will be placed on the same LUN as the .vmx file.

All changes made after the snapshot is taken are added to the new vmdk file(s). The delta VMDK files can grow until they reach the size of the original VMDK file.

If a snapshot exists for too long, this can generate problems as the SAN LUN can run out of disk space. If this happens, the VM’s will start to crash. Therefore, as a general rule of thumb, snapshots should not be left unattended for more than 3 days unless it is ensured that there is sufficient space on the data store.

If the snapshot is needed for a longer period, it is recommended to make a clone instead.

To check the size of the snapshot, simply browse datastore and look for a numbered vmdk file, e.g.

TestVM-0000001.vmdk

If a second snapshot is taken, it is named:

TestVM-0000002.vmdk

And so forth…