Adding, removing, growing and shrinking of VHDX files should be an online operation
Currently a VM has to be taken offline to add, remove, grow or shrink a VHDX. If the operating supports it this should be an online experience.
Future online capabilities should also be online in Windows Azure Pack
We are looking into this capability.
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Samir FARHAT (MVP) commented
Yes online capabilities must be added. I think that because Microsoft Azure is using VHD format till now, the expand property is only shown when the VM is offline, since WAP is derived form Microsoft Azure (sharing the same code)