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Change-Id: I3bed75a6cbbf616462d1c5efa29d42af7853ebe3 Reviewed-on: https://cl.snix.dev/c/snix/+/30253 Reviewed-by: Ilan Joselevich <personal@ilanjoselevich.com> Tested-by: besadii Autosubmit: Florian Klink <flokli@flokli.de>
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| title | description | date | lastmod | draft | weight | categories | tags | contributors | pinned | homepage |
|---|---|---|---|---|---|---|---|---|---|---|
| Snix as lower Nix Overlay Store | How to use Snix as your lower Nix Overlay Store | 2025-03-21T22:40:33+00:00 | 2025-03-21T22:40:33+00:00 | false | 51 | false | false |
Background
About a year ago, nix introduced a new experimental store type: Local Overlay Store. This store allows having multiple physical stores acting as a single logical store.
With this approach, stores are layered on top of each other with all but the top-most layer being writable, the others are read-only. The feature is described in the Upstream Nix Documentation. Work is ongoing to bring the feature to Lix aswell.
The main use-case is for it is having a large nix store mounted onto a machine
as read-only and configure nix to use data stored in it to avoid
rebuilding/substituting from cache.nixos.org.
Due to implementation details of this feature, the lower layer(s) can not only
be a location on the file system but also have another nix-daemon back the
lower layer(s).
Meet the new snix nix-daemon
Implementing nix-daemon protocol is a lot of effort, due to being entirely
custom and undocumented. On the other hand, mounting a large nix store is a
great fit for snix as our content-addressed store is much more
space efficient than conventional filesystem storage. A great example of this
can be found here.
What's interesting about Local Overlay Store, is that it uses only a small
subset of operations when talking to the nix-daemon. And this seemed like a
good opportunity to make using nix backed by our castore more seamless.
So we are happy to announce that as of today, snix has implemented all of
the operations required to operate as a lower layer in nix's overlay store.
Check out our [Guide]({{< ref "/docs/guides/local-overlay.md" >}}) on how to set it up.
Please test it out, let us know what you think and report bugs.
What's next for Local Overlay Store
We have plans expand our nix-daemon to support non-readonly mode and have all
possible substitutions go into your local castore and only use the upper nix
store for local builds.
We also want to make snix nix-daemon easier to use without having to run
multiple snix components.
Stay tuned.