I mistakenly deleted all snapshots and it is running now. When right clicking , the cancel task was grey out and I could not choose cancel. Why ?
You have initiated an operation that commits all the delta disk's blocks to the base disk - this means canceling this operation at any point could leave the base disk in an inconsistent state, which would result in a VM that could have its filesystem corrupt, refuse to boot, etc.
Hopefully this isn't anything critical - I wish you better luck next time
Thank you.
Is it impossible to cancel task just after issuing delete snapshot ?
Yes, Once you set things in motion, the blocks are starting to be merged and there is no way back. This also goes for "Consolidate Virtual Disks" options that searches for any stale snapshots and merges them to the base disk.