vSphere: Insufficient disk space on datastore

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Symptoms
A VMotion operation fails
When attempting to migrate, you see the error:

“Insufficient disk space on datastore ”

Cause
This issue may occur during a vMotion operation because vCenter Server 4.0 requires enough space to store two full-size copies of the virtual machine swap file simultaneously. If there is not enough space in the datastore, you receive this error.
For example, a virtual machine file with 8 GB of memory and a 2 GB reservation requires a swap that is 6 GB in size. You therefore temporarily require another 6GB, or a total of 12 GB.

Resolution
To resolve this issue, you must ensure that you have adequate amount of space on the datastores. To free up space, consider:

  • Increasing capacity
  • Moving virtual machines to other storage
  • Adding additional datastores
  • Removing old snapshots
  • Cleaning up VMware VMFS

For more information about these operations, see Issues when a datastore or VMFS volume is full or near capacity (1003412).

Workaround
Since only the non-reserved memory uses the swap file, you can temporarily increase the memory reservation of the virtual machine to match the allocated memory. this will disable the swapfile and free some space.