Q: How do I perform a dynamic resize of a clustered VHDX file?
A: Windows Server 2012 R2 introduced several nice additions to the VHDX. One is the ability to dynamically resize a VHDX file that was attached via the SCSI controller on the virtual machine; another is the ability to share a VHDX that's connected via the SCSI controller between multiple virtual machines, provided the VHDX is stored on a Cluster Shared Volume or a Scale-Out File Server (which actually uses CSV). The ability to share a VHDX file between multiple virtual machines enabled new guest clustering scenarios (in which a cluster is created between virtual machines) that required shared storage without having to use iSCSI or virtual Fibre Channel.
In Windows Server 2012 R2, it isn't possible to dynamically resize a VHDX file that has been marked as shared. To resize the VHDX, you must shut down all virtual machines that are using the VHDX file, resize the VHDX, then restart the virtual machines.