Write-Back Cache in Scale-Out File Server

My test environment setup consists of one Hyper-V server and a two-node Scale-Out File Server.  All have Windows Server 2012 R2 installed with the latest updates as of today.

My question is about Server 2012 R2's Write-Back Cache configured on disks created on the SOFS.  Is there any way to monitor the Write-Back Cache?  Or put another way, how can I tell how effective the current size (i.e. 5 GB) of the Write-Back Cache is?  How can I tell if I need more cache or that less could be used?

I'm thinking my Write-Back Cache for a virtual disk on my SOFS quickly becomes full, which is causing the file copy process (to a VM's VHDX running on that virtual disk) to look like hills and valleys--bursts of fast copy speeds interspaced with bursts of slow copy speeds.

Thanks!