Sometimes a VM will be writing to its disk faster than the VHD file can be merged with its parent. If that happens it can affect performance of other VMs on the host. If this goes on for too long it might be best to stop the operation until a time where it can be left alone long enough to finish.

To cancel a coalesce use this script.

/opt/xensource/sm/cleanup.py --uuid <sr-uuid> --abort

You can also set a flag to stop the server trying again

xe sr-param-set uuid=<sr-uuid> other-config:leaf-coalesce=false

Now you’ll be able to migrate the VDI and it won’t try to merge the VHD files first.