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>
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web/content/blog/2025-03-21-overlay-store/index.md
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web/content/blog/2025-03-21-overlay-store/index.md
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---
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title: "Using Snix as Lower Nix Overlay Store"
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description: "Using Snix as Lower Nix Store"
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date: 2025-03-21T22:40:33+00:00
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lastmod: 2025-03-21T22:40:33+00:00
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draft: false
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weight: 51
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categories: []
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tags: []
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contributors: []
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pinned: false
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homepage: false
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---
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## Background
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About a year ago, nix introduced a new experimental store type:
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[Local Overlay Store]. This store allows having multiple physical stores acting
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as a single logical store.
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With this approach, stores are layered on top of each other with all but the
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top-most layer being writable, the others are read-only. The feature is
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described in the [Upstream Nix Documentation][Local Overlay Store]. Work is
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[ongoing][lix-local-overlay] to bring the feature to Lix aswell.
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The main use-case is for it is having a large nix store mounted onto a machine
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as read-only and configure nix to use data stored in it to avoid
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rebuilding/substituting from `cache.nixos.org`.
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Due to implementation details of this feature, the lower layer(s) can not only
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be a location on the file system but also have another `nix-daemon` back the
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lower layer(s).
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## Meet the new `snix` nix-daemon
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Implementing `nix-daemon` protocol is a lot of effort, due to being entirely
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custom and undocumented. On the other hand, mounting a large nix store is a
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great fit for `snix` as our [content-addressed store][castore] is much more
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space efficient than conventional filesystem storage. A great example of this
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can be found [here][replit].
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What's interesting about Local Overlay Store, is that it uses only a small
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subset of operations when talking to the `nix-daemon`. And this seemed like a
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good opportunity to make using `nix` backed by our `castore` more seamless.
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So we are happy to announce that as of today, `snix` has implemented **all** of
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the operations required to operate as a lower layer in nix's overlay store.
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Check out our [Guide]({{\< ref "/docs/guides/local-overlay" >}}) on how to set
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it up.
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Please test it out, let us know what you think and report [bugs].
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## What's next for Local Overlay Store
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We have plans expand our `nix-daemon` to support non-readonly mode and have all
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possible substitutions go into your local `castore` and only use the upper nix
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store for local builds.
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We also want to make `snix` nix-daemon easier to use without having to run
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multiple `snix` components.
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Stay tuned.
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[castore]: https://snix.dev/docs/components/overview/
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[bugs]: https://git.snix.dev/snix/snix/issues
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[Local Overlay Store]: https://nix.dev/manual/nix/2.26/store/types/experimental-local-overlay-store.html
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[replit]: https://blog.replit.com/tvix-store
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[lix-local-overlay]: https://gerrit.lix.systems/c/lix/+/2859
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web/content/docs/guides/local-overlay.md
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web/content/docs/guides/local-overlay.md
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---
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title: "Use as a lower store with nix"
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slug: local-overlay
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description: ""
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summary: ""
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date: 2025-03-21T22:40:33+00:00
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lastmod: 2025-03-21T22:40:33+00:00
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draft: false
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weight: 14
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toc: true
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---
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This document describes how to configure `snix` as the lower layer in your
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[Local Overlay] nix store.
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### Build required `snix` components
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To use this feature you will need to 2 `snix` compontents, for detailed building
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instructions see [Building]({{< ref "building" >}}).
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```console
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$ nix-build -A snix.store
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$ nix-build -A snix.nix-daemon
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```
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These will provide `snix-store` and `nix-daemon` binaries.
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### Run the `snix` daemon
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`snix daemon` is the component exposing `castore` and `store` data. By default,
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these live inside `/var/lib/snix`, so make sure it's writable for the user
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you're executing it with. See `snix-store daemon --help` for customization
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options. `/var/lib/snix`, you can run `snix-store daemon --help` for
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customization instructions.
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You can run the daemon with:
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```console
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$ $(nix-build -A snix.snix-store)/bin/snix-store daemon
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```
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### Mount the castore onto your file system
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To expose the store paths and their contents as a file system, if can be
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FUSE-mounted with the following command:
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```console
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$ $(nix-build -A snix.snix-store)/bin/snix-store mount /path/to/mount
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```
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This mount will talk to the previously invoked daemon.
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Note that by default, this mount won't allow listing files and directories at
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the root of the store, if you want to enable it, use the `--list-root` flag, but
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be careful with it if your store is really large.
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### Run `snix` nix-daemon
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```console
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$ $(nix-build -A snix.nix-daemon)/bin/nix-daemon -l /tmp/snix-daemon.sock \
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--unix-listen-unlink
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```
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This will launch the `snix` nix-daemon listening on a unix domain socket
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### Create an overlayfs mount
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Bind mount your real /nix store on the side, so that nix has direct access to
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it, this is optional but allows you to have access to your real nix store
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without unmounting:
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```console
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$ mount --bind /nix /opt/nix
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```
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```console
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$ mount -t overlay overlay \
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-o lowerdir=/path/to/mount \
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-o upperdir=/opt/nix \
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/nix
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```
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### Configure nix to use the daemon
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With all of the above out of the way, we are ready to configure nix. In the
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proposed setup we will configure nix-daemon with an overlay store but for the
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Nix CLI you can just configure nix with the overlay store.
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#### nix-daemon
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The daemon can be configured in the following way:
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Add the following line to your `/etc/nix.conf`
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```
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store = local-overlay://?state=/opt/nix/var/nix&upper-layer=/opt/nix/store&check-mount=false&lower-store=unix%3A%2F%2F%2Ftmp%2Fsnix-daemon.sock
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```
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#### Personal nix config
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With the above configuration in your `/etc/nix.conf`, we need to tell nix not to
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use it but instead use `store = daemon` so that only the daemon is aware of the
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Local Overlay Store.
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This can be achieved by either setting the env variable
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`NIX_CONFIG='store = daemon` or by adding `store = daemon` to your
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`$XDG_CONFIG_HOME/nix.conf` file.
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### Profit
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With the above setup you should now be able to have nix use Snix Castore as its
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lower store.
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Note that snix's FUSE mount might be performing slower than the native
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file-system depending on your workload. Please file bugs if you notice obviously
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bad performance.
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[local overlay]: https://nix.dev/manual/nix/2.26/store/types/experimental-local-overlay-store.html
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