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A virtual machine cannot boot after extending a base virtual disk that is part of a snapshot hierarchy

Details

  • A virtual machine fails to power on if you extend the base disk of a snapshot which is part of a redo hierarchy
  • You see the error:

    The parent virtual disk has been modified since the child was created

Solution

This issue may occur if the size of the base disk does not match the logical size of the redo log.
 
To determine the original size of the base disk, look at the .vmdk file corresponding to the delta disk redo log. For example, an entry for a delta disk in the .vmdk file looks like:
RW 2048 VMFSSPARSE "foo-43489354-delta.vmdk"

Note: The disk size is expressed in number of sectors. In this example, the disk size is 2048, or 1MB.
 
The corresponding base disk descriptor file foo.vmdk looks like:
# Extent description
RW 2048 VMFS "foo-flat.vmdk"
If you extend the base disk to 2MB, the foo.vmdk file looks like:
# Extent description
RW 4096 VMFS "foo-flat.vmdk"

To reset the logical size of the base disk, replace 4096 with 2048 so that the foo.vmdk file looks like:
# Extent description
RW 2048 VMFS "foo-flat.vmdk"
 
Note: Additional datastore space is required to clone the disk

There are two options to fix this issue:
  1. Reset the base disk back to the original size in the disk descriptor file.
  2. Keep the new base disk size, and change ALL associated delta disk descriptor files to the new size.
Use this procedure to modify the virtual machine disk properties:
  1. Shutdown the virtual machine.
  2. Back-up each vmdk descriptor file in turn, and modify the RW value(s) as requires as detailed above. For example, base vmdk descriptor files, or all associated snapshot descriptor files.
  3. Clone the disk from the most recent snapshot to a new virtual disk.

    Note    : Clone the disk, so that in the event of there being an issue, the original file(s) are available.

  4. Using Edit Settings, remove the existing disk from the virtual machine only (do not remove from disk), and add the new disk to virtual machine.
  5. Power on the virtual machine.
  6. When the size has been verified within the virtual machine, delete the original disk and associated snapshot files after confirming the virtual machine is running as expected.

Tags

cannot-power-on power-on-fails power-on-vm vm-power-on-fails vm-snapshot

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