We use the nbdkit-cacheextents-filter when converting from HTTPS, SSH and VDDK sources. However I don't believe this is a good idea in current virt-v2v for the reasons outlined in this commit message:

https://github.com/libguestfs/virt-v2v/commit/48c4ce8e6cf6f1c390a48245ef0f99233f80cfe8

mkletzan@redhat.com Please check out my reasoning there. You wrote that filter and I think made the change to virt-v2v. It wasn't wrong then, but I think it's wrong to use it now.