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/docs/guides/local-overlay.md

@@ -0,0 +1,118 @@
+---
+title: "Use as a lower store with nix"
+slug: local-overlay
+description: ""
+summary: ""
+date: 2025-03-21T22:40:33+00:00
+lastmod: 2025-03-21T22:40:33+00:00
+draft: false
+weight: 14
+toc: true
+---
+
+This document describes how to configure `snix` as the lower layer in your
+[Local Overlay] nix store.
+
+### Build required `snix` components
+
+To use this feature you will need to 2 `snix` compontents, for detailed building
+instructions see [Building]({{< ref "building" >}}).
+
+```console
+$ nix-build -A snix.store
+$ nix-build -A snix.nix-daemon
+```
+
+These will provide `snix-store` and `nix-daemon` binaries.
+
+### Run the `snix` daemon
+
+`snix daemon` is the component exposing `castore` and `store` data. By default,
+these live inside `/var/lib/snix`, so make sure it's writable for the user
+you're executing it with. See `snix-store daemon --help` for customization
+options. `/var/lib/snix`, you can run `snix-store daemon --help` for
+customization instructions.
+
+You can run the daemon with:
+
+```console
+$ $(nix-build -A snix.snix-store)/bin/snix-store daemon
+```
+
+### Mount the castore onto your file system
+
+To expose the store paths and their contents as a file system, if can be
+FUSE-mounted with the following command:
+
+```console
+$ $(nix-build -A snix.snix-store)/bin/snix-store mount /path/to/mount
+```
+
+This mount will talk to the previously invoked daemon.
+
+Note that by default, this mount won't allow listing files and directories at
+the root of the store, if you want to enable it, use the `--list-root` flag, but
+be careful with it if your store is really large.
+
+### Run `snix` nix-daemon
+
+```console
+$ $(nix-build -A snix.nix-daemon)/bin/nix-daemon -l /tmp/snix-daemon.sock \
+    --unix-listen-unlink
+```
+
+This will launch the `snix` nix-daemon listening on a unix domain socket
+
+### Create an overlayfs mount
+
+Bind mount your real /nix store on the side, so that nix has direct access to
+it, this is optional but allows you to have access to your real nix store
+without unmounting:
+
+```console
+$ mount --bind /nix /opt/nix
+```
+
+```console
+$ mount -t overlay overlay \
+    -o lowerdir=/path/to/mount \
+    -o upperdir=/opt/nix \
+    /nix
+```
+
+### Configure nix to use the daemon
+
+With all of the above out of the way, we are ready to configure nix. In the
+proposed setup we will configure nix-daemon with an overlay store but for the
+Nix CLI you can just configure nix with the overlay store.
+
+#### nix-daemon
+
+The daemon can be configured in the following way:
+
+Add the following line to your `/etc/nix.conf`
+
+```
+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
+```
+
+#### Personal nix config
+
+With the above configuration in your `/etc/nix.conf`, we need to tell nix not to
+use it but instead use `store = daemon` so that only the daemon is aware of the
+Local Overlay Store.
+
+This can be achieved by either setting the env variable
+`NIX_CONFIG='store = daemon` or by adding `store = daemon` to your
+`$XDG_CONFIG_HOME/nix.conf` file.
+
+### Profit
+
+With the above setup you should now be able to have nix use Snix Castore as its
+lower store.
+
+Note that snix's FUSE mount might be performing slower than the native
+file-system depending on your workload. Please file bugs if you notice obviously
+bad performance.
+
+[local overlay]: https://nix.dev/manual/nix/2.26/store/types/experimental-local-overlay-store.html