If you encounter the error “the hot-plug operation failed” when you add memory it is sure that the Lun on which is the SWAP file of the machine is full!
In my case, I wanted to add hot memory on my virtual machine when I encounter this error:
Failed to extend swap file from 16777216 KB to 67108864 KB.
Current swap file size is 16777216 KB.
FSR [0:1498050061] Unable to pre-grow the swap file from 16777216 KB to 67108864 KB for source virtual machine 1069502 with No space left on device.
Failed waiting for data. Error 195887319. No space left on device.
Hot-add of memory failed.
The hot-plug operation failed.
This happens simply because when creating a virtual machine, a SWAP file is created (yourVM.vswp) and this file corresponds to the size allocated to the memory of the virtual machine.
So if you increase the size of your memory, then the swap file will also increase.
To expand a datastore, you can see my “How to” that explain How to – Increase VMFS datastore with vSphere Web Client
I hope this post help you, don’t hesitate to post a comment if you have questions.