[djs55]

We'll probably use AHCI since hyperkit https://github.com/docker/hyperkit (the Mac hypervisor based on xhyve based on bhyve) lacks a virtio-scsi implementation at the moment. When the VM sends a TRIM ("these blocks aren't used any more, you can free them") we need to make use of this on the host. On Linux/BSD you could tell the host OS that the blocks aren't needed in the file using something like FALLOC_FL_PUNCH_HOLE and the kernel would take care of it e.g. by send TRIMs to the real storage device or shuffling filesystem metadata around. Unfortunately the Mac filesystems in common use don't support this so we'll need to manage this ourselves, e.g. by moving blocks from the end of the file into the middle and shrinking the file.