1a78270f54
It seems this is its own Markdown context of some sort, so that hyperlink url needs to be moved inside to work. Change-Id: Id35fdf97ccc7a5ed087360d27a2cda3a7df0c850 Reviewed-on: https://cl.snix.dev/c/snix/+/30265 Tested-by: besadii Reviewed-by: Vova Kryachko <v.kryachko@gmail.com> Autosubmit: Florian Klink <flokli@flokli.de>
140 lines
4.1 KiB
Markdown
140 lines
4.1 KiB
Markdown
---
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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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$ $(nix-build -A snix.snix-store)/bin/snix-store daemon
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```
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### Mount the store
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To expose the store paths and their contents as a file system, if can be
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FUSE-mounted with the following command:
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```console
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$ $(nix-build -A snix.snix-store)/bin/snix-store mount /path/to/mount
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```
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This mount will talk to the previously invoked daemon.
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Note that by default, this mount won't allow listing files and directories at
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the root of the store, if you want to enable it, use the `--list-root` flag, but
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be careful with it if your store is really large.
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### Run `snix` nix-daemon
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```console
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$ $(nix-build -A snix.nix-daemon)/bin/nix-daemon -l /tmp/snix-daemon.sock \
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--unix-listen-unlink
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```
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This will launch the `snix` nix-daemon listening on a unix domain socket.
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Nix will communicate with it to get metadata about store paths.
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### Create an overlayfs mount
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{{<callout>}}
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Depending on your usecase, this might not be appropriate for a physical NixOS
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system, replacing `/nix` globally.
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In these cases, you want to ensure store paths needed to boot the system are
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available on the plain disk.
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You most likely want to mount this combined mountpoint elsewhere, and spin up a
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separate mount namespace (via `systemd-nspawn`, `bwrap` or similar) exposing it
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at `/nix` in there. You have been warned!
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{{</callout>}}
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Bind mount your real /nix store on the side, so that nix has direct access to
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it, this is optional but allows you to have access to your real nix store
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without unmounting:
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```console
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$ 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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{{<callout>}}
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There are some known (and not yet worked-on) performance issues in Snix
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castore, which is why the mount is expected to perform slower than the native
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file-system. Depending on your workload, this might or might not be an issue.
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Check [our Bug tracker][castore-perf-issues] for updates on that topic.
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[castore-perf-issues]: https://git.snix.dev/snix/snix/issues?q=&type=all&sort=&labels=14%2c24
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{{</callout>}}
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[local overlay]: https://nix.dev/manual/nix/2.26/store/types/experimental-local-overlay-store.html
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