We are proud to announce the latest release of Unikraft, v0.20.0.
This release comes with the much anticipated redesign and implementation of VFS, which brings enhanced performance, better maintainability, and improved compatibility with modern applications. It also contains plenty of fixes that improve the stability and application support of Unikraft.
In this blog post, we present the new features available in Unikraft. For a full breakdown, please check out the changelog.
DEPRECATION NOTICEThe old VFS, vfscore, will be deprecated in Unikraft 0.22.0 (early 2026). For help on transitioning, consult the migration guide or join us on Discord.
COMPATIBILITY NOTICEThe implementation of
arch_prctlsystem call has been migrated fromapp-elfloadertolibposix-process. Configurations usingCONFIG_APPELFLOADER_ARCH_PRCTLshould now useCONFIG_LIBPOSIX_PROCESS_ARCH_PRCTL.
This release marks a milestone in the ongoing effort to replace vfscore and modernize the Unikraft file and filesystem stack with the introduction of ukfs -- the Unikraft filesystem interface -- as well as a new full-stack VFS implementation.
Building upon the ukfile interface introduced in 0.16, ukfs exists as a separation layer between filesystem drivers and higher level consumers.
Filesystem nodes exist as special instances of ukfiles, integrating seamlessly with existing filesystem-agnostic code, such as posix-fd*, while providing the ukfs API for interested consumers.
A key such consumer is posix-vfs, a core library providing the familiar *nix virtual filesystem (VFS) anchored at /, along with all related state, operations, and syscalls.
On the other end of the stack this release also introduces ukfs-ramfs, a memory-resident volatile filesystem, replacing legacy vfscore ramfs, along with uksparsebuf and ukpod, two internal utility libraries supporting file drivers.
Among the many performance and compatibility benefits of the new VFS stack and ramfs, a few notable ones are: sparse files, multiple hardlinks, bind mounts, and mounting on top of a non-empty directory (shadowing its previous contents until unmount).
As a consequence of the extensive rewrite done across the past four releases, the responsibilities of vfscore are now handled by dedicated libraries in the ukfile/ukfs stack:
ukfile -- core abstraction for "a file"ukfs -- core abstraction for "a filesystem node"posix-fd -- open file descriptions, state held by open filesposix-fdio -- POSIXey I/O & control operations on open filesposix-fdtab -- file descriptors & their managementposix-vfs -- the filesystem (VFS) anchored at root, everything to do with pathsposix-vfs-fstab -- constructing the VFS at bootAll had their README.md files updated to better describe their design and interactions.
Consult these for a more in-depth overview of the new file(system) stack.
All changes relating to the new VFS stack are opt-in, with existing configs that use vfscore continuing to work.
We encourage users to migrate their configurations as soon as practical, as vfscore is likely to be deprecated in the near future. This can be easily done following these steps:
Replace all hard dependencies to vfscore with soft dependencies in your application’s Config.uk (select LIBVFSCORE → imply LIBVFSCORE)
LIB9PFS, LIBDEVFS, LIBRAMFS)From interactive config prompt (e.g., make menuconfig):
Disable vfscore.
Enable posix-vfs; under its menu:
CONFIG_LIBPOSIX_VFS_SYSCALLS)Enable posix-vfs-fstab; under its menu:
/devNOTE: Filesystems other than ramfs & devfs are not yet supported. Continue using the vfscore stack for now.
Application support has improved based on the updates of the Unikraft core.
And together with additions and improvements to the catalog and catalog-core repositories.
An important milestone is the now functional support of the Java runtime.
Unikraft Summer Workshop 2025 (USW'25), a free and virtual workshop held by members of the Unikraft community, took part for 3 weeks, between June 23 and July 4, 2025. It consisted of 6 sessions filled with tutorials and workshops on how to configure, build, run and debug applications using Unikraft.
The final hackathon took place on Saturday, 12 July 2025, 9am-5pm CEST.
22 participants, split into 6 teams, contributed to the catalog-core and catalog repositories, and to various libraries and other components of the Unikraft ecosystem.
GSoC'25 projects are almost done. We are very happy with the results of our GSoC participants. Check out their work products:
Prasoon Kumar from Indian Institute of Technology Bombay in Mumbai, India
Abolfazl Soltani from Vrije Universiteit Amsterdam in Amsterdam, The Netherlands
Shashank Srivastava from GL Bajaj Institute of Technology and Management in Greater Noida, India
Thanks so much for your involvement! And keep contributing! 🤝
Feel free to ask questions, report issues, and meet new people.