Using virtio-fs on a unikernel

03 Nov 2020 — by Fotis Xenakis

This article provides an overview of virtio-fs, a novel way for sharing the host file system with guests and OSv, a specialized, lightweight operating system (unikernel) for the cloud, as well as how these two fit together.


Virtio-fs is a new host-guest shared filesystem, purpose-built for local file system semantics and performance. To that end, it takes full advantage of the host’s and the guest’s colocation on the same physical machine, unlike network-based efforts, like virtio-9p.

As the name suggests, virtio-fs builds on virtio for providing an efficient transport: it is included in the (currently draft, to become v1.2) virtio specification as a new device. The protocol used by the device is a slightly extended version of FUSE, providing a solid foundation for all file system operations native on Linux. Implementation-wise, on the QEMU side, it takes the approach of splitting between the guest interface (handled by QEMU) and the host file system interface (the device “backend”). The latter is handled by virtiofsd (“virtio-fs daemon”), running as a separate process, utilizing the vhost-user protocol to communicate with QEMU.

One prominent performance feature of virtio-fs is the DAX (Direct Access) window. It’s a shared memory window between the host and the guest, exposed as device memory (a PCI BAR) to the second. Upon request, the host (QEMU) maps file contents to the window for the guest to access directly. This bears performance gains due to taking VMEXITs out of the read/write data path and bypassing the guest page cache on Linux, while not counting against the VM’s memory (since it’s just device memory, managed on the host).

virtio-fs DAX architecture

Virtio-fs is under active development, with its community focussing on a pair of device implementation in QEMU and device driver in Linux. Both components are already available upstream in their initial iterations, while upstreaming continues further e.g. with DAX window support.


OSv is a unikernel (framework). The two defining characteristics of a unikernel are:

  • Application-specialized: a unikernel is an executable machine image, consisting of an application and supporting code (drivers, memory management, runtime etc.) linked together, running in a single address space (typically in guest “kernel mode”).
  • Library OS: each unikernel only contains the functionality mandated by its application in terms of non-application code, i.e. no unused drivers, or even whole subsystems (e.g. networking, if the application doesn’t use the network).

OSv in particular strives for binary compatibility with Linux, using a dynamic linker. This means that applications built for Linux should run as OSv unikernels without requiring modifications or even rebuilding, at least most of the time. Of course, not the whole Linux ABI is supported, with system calls like fork() and relatives missing by design in all unikernels, which lack the notion of a process. Despite this limitation, OSv is quite full featured, with full SMP support, virtual memory, a virtual file system (and many filesystem implementations, including ZFS) as well as a mature networking stack, based on the FreeBSD sources.

At this point, one is sure to wonder “Why bother with unikernels?”. The problem they were originally introduced to solve is the bloated software stack in modern cloud computing. Running general-purpose operating systems as guests, typically for a single application/service, on top of a hypervisor which already takes care of isolation and provides a standard device model means duplication, as well as loss of efficiency. This is were unikernels come in, trying to be just enough to support a single application and as light-weight as possible, based on the assumption that they are executing inside a VM. Below is an illustration of the comparison between general-purpose OS, unikernels and containers (as another approach to the same problem, for completeness).

Unikernels vs GPOS vs containers

OSv, meet virtio-fs

As is apparent e.g. from the container world, it is very common for applications running in isolated environments (such as containers, or unikernels even more so) to require host file system access. Whereas containers sharing the host kernel thus have an obvious, controlled path to the host file system, with unikernels this has been more complex: all solutions were somewhat heavyweight, requiring a network link or indirection through network protocols. Virtio-fs then provided a significantly more attractive route: straight-forward mapping of fs operations (via FUSE), reusing the existing virtio transport and decent performance without high memory overhead.

The OSv community quickly identified the opportunity and came up with a read-only implementation on its side, when executing under QEMU. This emphasized being lightweight complexity-wise, while catering to many of its applications’ requirements (they are stateless, think e.g. serverless). Notably, it includes support for the DAX window (even before that’s merged in upstream QEMU), providing excellent performance, directly rivalling that of its local (non-shared) counterparts such as ZFS and ROFS (an OSv-specific read-only file system).

One central point is OSv’s support for booting from virtio-fs: this enables deploying a modified version or a whole new application without rebuilding the image, just by adjusting its root file system contents on the host. Last, owing to the DAX window practically providing low-overhead access to the host’s page cache, scalability is also expected to excel, with it being a common concern due to the potentially high density of unikernels per host.

For example, to build the cli OSv image, bootable from virtio-fs, using the core OSv build system:

scripts/build fs=virtiofs export=all image=cli

This results in a minimal image (just the initramfs), while the root fs contents are placed in a directory on the host (build/export here, by default).

Running the above image is just a step away (may want to use the virtio-fs development version of QEMU, e.g. for DAX window support):

scripts/ --virtio-fs-tag=myfs --virtio-fs-dir=$(pwd)/build/export

This orchestrates running both virtiofsd and QEMU, using the contents of build/export as the root file system. Any changes to this directory, directly from the host will be visible in the guest without re-running the previous build step.


OSv has gained a prominent new feature, powered by virtio-fs and its QEMU implementation. This allows efficient, lightweight and performant access to the host’s file system, thanks to the native virtio transport, usage of the FUSE protocol and the DAX window architecture. In turn, it enables use cases like rapid unikernel reconfiguration.