clonevm faster: make use of --reflink

[rugubara]

I'm running vbox machines off a btrfs partition with CoW. When combined with bcache (SSD-backed cache) the fragmentation penalty of fragmented images doesn't hurt much - i.e. I get adequate perfomance of 3-8 VMs on my Lenovo T540p notebook.

When I clone the vm (separate copy), virtualbox does a full copy of the image - taking considerable time (4 hours for 100GB image) and space. I suggest to utilize the "cp --reflink=auto" instead. The copy is made instantly (if possible and falls back to traditional copy) and no additional space is taken until there are changed blocks in the image file.

[Another]

This applies to macOS as well.

-c copy files using clonefile(2)