When I was scaling up my home lab, I started thinking more about data management. I hadn’t (and still haven’t) set up any form of network storage. I have, however, set up a backup mechanism using Borg. Still, I want to operate lots of virtual machines, and backing up each one of them separately seemed excessive. So I started thinking, what if I just let the host machines back up the data? After all, the amount of physical hosts I have in my home lab is unlikely to increase drastically.

The Use Case for Sharing Directories

I started working out this idea further. Without network storage, I needed a way for guest VMs to access the host’s disks. Here there are two possibilities, either expose some block device or a file system. Creating a whole virtual disk for just the data of some VMs seemed wasteful, and from my experiences also increases backup times dramatically. I therefore searched for a way to mount a directory from the host OS on the guest VM. This is when I stumbled upon this blog post talking about sharing directories with virtual machines.

Sharing Directories with virtio-9p

virtio-9p is a way to map a directory on the host OS to a special device on the virtual machine. In virt-manager, it looks like the following: Under the hood, virtio-9p uses the 9pnet protocol. Originally developed at Bell Labs, support for this is available in all modern Linux kernels. If you share a directory with a VM, you can then mount it. Below is an extract of my /etc/fstab to automatically mount the directory:

data	/mnt/data	9p	trans=virtio,rw	0	0

The first argument (data) refers to the name you gave this share from the host With the trans option we specify that this is a virtio share.

Problems with virtio-9p

At first I had no problems with my setup, but I am now contemplating just moving to a network storage based setup because of two problems.

The first problem is that some files have suddenly changed ownership from libvirt-qemu to root. If the file is owned by root, the guest OS can still see it, but cannot access it. I am not entirely sure the problem lies with virtio, but I suspect it is. For anyone experiencing this problem, I wrote a small shell script to revert ownership to the libvirt-qemu user:

find -printf "%h/%f %u\n"  | grep root | cut -d ' ' -f1 | xargs chown libvirt-qemu:libvirt-qemu

Another problem that I have experienced, is guests being unable to mount the directory at all. I have only experienced this problem once, but it was highly annoying. To fix it, I had to reboot the whole physical machine.

Alternatives

virtio-9p seemed like a good idea, but as discussed, I had some problems with it. It seems virtioFS might be a an interesting alternative as it is designed specifically for sharing directories with VMs.

As for me, I will probably finally look into deploying network storage either with NFS or SSHFS.