For example, if I'm running 5 VM's, there is a good chance that many of the pages are identical. Not only do I want those pages to be deduplicated, but I want them to be zero-copy (ie. not deduplicated after-the-fact by some daemon).
To do that, the guest block cache needs to be integrated with the host block-cache, so that whenever some guest application tries to map data from disk, the host notices that another virtual machine has already caused this data to be loaded, so we can just map the same page of already loaded data into the VM that is asking.