feat(blog): Add nix-deamon blog and guide.

Change-Id: I46b8632ac2bfa43ccf95bd63d37e08a8f6b0869e
Reviewed-on: https://cl.snix.dev/c/snix/+/30244
Autosubmit: Vova Kryachko <v.kryachko@gmail.com>
Tested-by: besadii
Reviewed-by: Florian Klink <flokli@flokli.de>

web/content/blog/2025-03-21-overlay-store/index.md

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+---
+title: "Using Snix as Lower Nix Overlay Store"
+description: "Using Snix as Lower Nix Store"
+date: 2025-03-21T22:40:33+00:00
+lastmod: 2025-03-21T22:40:33+00:00
+draft: false
+weight: 51
+categories: []
+tags: []
+contributors: []
+pinned: false
+homepage: 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][Local Overlay Store]. Work is
+[ongoing][lix-local-overlay] 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][castore] is much more
+space efficient than conventional filesystem storage. A great example of this
+can be found [here][replit].
+
+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" >}}) 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.
+
+[castore]: https://snix.dev/docs/components/overview/
+[bugs]: https://git.snix.dev/snix/snix/issues
+[Local Overlay Store]: https://nix.dev/manual/nix/2.26/store/types/experimental-local-overlay-store.html
+[replit]: https://blog.replit.com/tvix-store
+[lix-local-overlay]: https://gerrit.lix.systems/c/lix/+/2859