So I decided to try and improve the performance of one of my VMs that was running torrents.
Instead of using samba to mount the filesystem from inside the guest I went with QEMU / KVM's v9fs.
The filesystem is actually called 9p or Plan 9 File System:
After mounting and figuring out permission issues I started rtorrent and got the error
[File chunk write error: Invalid argument.]
Apparently rtorrent uses mmap and the Plan9 file system kernel module did not support memory mapping. However it was added in a later kernel release. So all I had to do was upgrade my kernel (I was on the default 14.04 LTS kernel):
sudo apt-get install linux-image-generic-lts-vivid linux-headers-generic-lts-vivid
Restarted and used the following mount command it all worked:
mount -t 9p -o trans=virtio,version=9p2000.L,cache=mmap,rw /mount/external /data
Before the cache=mmap did not work (only cache=loose or cache=none)