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Wodan is a flash friendly, safe and flexible filesystem library for Mirage OS.

It provides a key-value store as well as an Irmin backend.

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Wodan works, but still needs more hardening and more testing in concurrent environments.

The store it provides is usable for basic tasks, but Wodan itself doesn't provide ways to serialize complex objects or deal with non-fixed size keys or values larger than 64k. You are expected to layer a higher-level store such as Irmin on top of it for such amenities.

To get the best performance out of Wodan, you are also expected to understand some of the tradeoffs involved in flushing data to disk and picking a block size.


Here is a Wodan for Mirage users presentation.

Unikernel usage is best explained through an example.

See and the README file it contains for an overview.

There is also an ICFP 2017 presentation which is more focused on the internals.


This explains some of the design choices behind Wodan.

ICFP 2017

Building, installing and running

Wodan requires Opam 2, Dune, Mirage 3, and OCaml 4.08 through 4.11.

An opam switch with flambda is recommended for performance reasons.

opam switch 4.11.1+fp+flambda

Building the library, CLI, and Irmin bindings

make deps
# Follow the opam instructions

CLI usage

./wodanc --help

If wodan-unix has been installed (or pinned) through Opam, you can instead type:

wodanc --help

When developping, you may prefer to use the following for immediate feedback on any changes:

dune exec src/wodan-unix/wodanc.exe

At the moment the CLI supports creating filesystems, dumping and restoring data into them, plus some more specialised features explained below.

Creating an empty disk

In order to be able to use the following commands, you need a disk for Wodan to operate on.

The following commands create a zeroed image of size 512M named disk.img.

touch disk.img
fallocate -z -l 512m disk.img


./wodanc bench

Running tests

make test
./wodanc exercise

Running American Fuzzy Lop (AFL)

This requires OCaml compiled with AFL support.

opam switch 4.11.1+afl
sudo sysctl kernel.core_pattern=core
echo performance |sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
make fuzz


You can rely on the CI for some checks, you can also run tests with make test and ensure the code is well formatted by running make ocamlformat before any commit.


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