Storage vMotion

How to change the thick or thin provisioning of a virtual disk

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🟢 Converting Thin to Thick (Inflation)

If your datastore has plenty of space and you need to eliminate the performance “write penalty” of a thin disk, you can Inflate it.

  1. Preparation: Power off the VM. Ensure there are no snapshots attached to the VM, as inflation only works on the base .vmdk.
  2. Locate the File: Go to the VM’s Summary tab. Under Resources, right-click the datastore and select Browse Datastore.
  3. The “Inflate” Action: Open the VM folder and find the .vmdk file. Right-click it and select Inflate.
    • Note: If “Inflate” is greyed out, the VM is likely still powered on or is already thick-provisioned.
  4. Finalize: Once the task finishes, you may need to Reload the .vmx file to ensure the vSphere UI reflects the new “Thick” status.

🔵 Converting Thick to Thin (Migration)

Converting back to Thin provisioning is slightly more complex because you cannot “deflate” a disk in place. You must move the data to a new location to reclaim the space.

Method A: Migration (Requires two Datastores)

This is the cleanest way to convert a disk using Storage vMotion or an offline migration.

  1. Power Off the VM (required for standard Migration; not required for Storage vMotion if licensed).
  2. Migrate: Right-click the VM and select Migrate > Change Datastore.
  3. Select Format: In the migration wizard, look for the Select Virtual Disk Format dropdown and choose Thin Provision.
  4. Target: Select a different datastore than the current one. vSphere will copy the blocks, only writing the actual data to the destination, effectively “thinning” the disk.

Method B: Cloning (Single Datastore)

If you only have one datastore, you cannot migrate the VM to itself to change the format.

  1. Right-click the VM and select Clone.
  2. During the clone wizard, select your current datastore as the destination.
  3. In the Disk Format section, select Thin Provision.
  4. Once complete, delete the old “Thick” VM and keep the new “Thin” one.

⚠️ Important Considerations

  • Backups: Always have a fresh backup. Moving or inflating disks is a heavy I/O operation.
  • Lazy vs. Eager Zeroed: When inflating to Thick, vSphere usually defaults to “Lazy Zeroed” (space is reserved, but blocks aren’t cleared until written to). For maximum performance (e.g., for Database logs or VSAN), “Eager Zeroed” is preferred.
  • Space Check: Before converting from Thin to Thick, ensure your datastore can handle the immediate consumption of the entire disk size.

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