diff --git a/web/content/intro-nix/_index.md b/web/content/intro-nix/_index.md
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+---
+title: "Intro to nix"
+date: 2023-09-15
+---
+
+Basic introduction to Nix for users of the jungle machines. You should be able
+to access the jungle machines, otherwise [request access](/access).
diff --git a/web/content/intro-nix/ch1.md b/web/content/intro-nix/ch1.md
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+---
+title: "Chapter 1: Packages"
+description: "Here we show where packages come from"
+date: 2023-06-13T19:36:57+02:00
+weight: 1
+---
+
+In this chapter we describe where the packages available in the cluster come
+from and how to load them.
+
+## Where packages come from
+
+The packages in the jungle cluster are constructed by *layers*. Each layer
+applies some changes over the previous one:
+
+1. The first layer is [nixpkgs][1], a large repository of packages maintained by
+   the NixOS community. It provides packages like gcc, bash, gcc or the linux
+   kernel.
+
+[1]: https://github.com/NixOS/nixpkgs/
+
+2. The second layer is [bscpkgs][2], it takes the nixpkgs set of packages and
+   expands it by adding custom packages from the BSC such as Nanos6, nOS-V,
+   NODES, ovni or wxparaver.
+
+[2]: https://pm.bsc.es/gitlab/rarias/bscpkgs
+
+3. The third layer is [jungle][3], it takes the extended packages from bscpkgs
+   and configures them for the jungle cluster. For example, we configure MPICH
+   to use the OmniPath network and set it as the default implementation.
+
+[3]: https://pm.bsc.es/gitlab/rarias/jungle
+
+These layers are called *overlays* in Nix and they are the default mechanism
+used to modify the packages. Generally you will use the packages defined in the
+last layer (jungle) but you can define your own additional layer to specify
+custom changes. For example, instead of choosing MPICH, you may want to use
+Intel MPI instead by default.
+
+## Loading packages in an ephemeral shell
+
+You can manually load packages in a *new* shell with `nix shell jungle#<pkg>`,
+for example:
+
+```
+hut% which ovniemu
+ovniemu not found
+hut% nix shell jungle#bsc.ovni
+hut% which ovniemu
+/nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0/bin/ovniemu
+hut% exit
+hut%
+```
+
+You can also specify multiple packages by listing them as parameters of `nix
+shell`:
+
+```
+hut% nix shell jungle#bsc.ovni jungle#bsc.osumb
+hut% which osu_bw
+/nix/store/lnjirzllhjn2fadlqzrz7a547iawl8jc-osu-micro-benchmarks-7.1-1/bin/osu_bw
+hut% exit
+```
+
+Or make the bash (zsh in this case) shell expand them:
+
+```
+hut% echo nix shell jungle#bsc.{ovni,osumb}
+nix shell jungle#bsc.ovni jungle#bsc.osumb
+hut% nix shell jungle#bsc.{ovni,osumb}
+hut% which osu_bw
+/nix/store/lnjirzllhjn2fadlqzrz7a547iawl8jc-osu-micro-benchmarks-7.1-1/bin/osu_bw
+hut% exit
+```
+
+You can use TAB to see which packages are available:
+
+```
+hut% nix shell jungle#bsc.n<TAB>
+jungle\#bsc.nanos6              jungle\#bsc.nixtools
+jungle\#bsc.nanos6Debug         jungle\#bsc.nix-wrap
+jungle\#bsc.nanos6Git           jungle\#bsc.nodes
+jungle\#bsc.nanos6GlibcxxDebug  jungle\#bsc.nodesGit
+jungle\#bsc.nanos6-icc          jungle\#bsc.nodesRelease
+jungle\#bsc.nanos6-icx          jungle\#bsc.nodesWithOvni
+jungle\#bsc.nanos6Release       jungle\#bsc.nosv
+jungle\#bsc.nix-mn4
+```
+
+Notice that these packages are evaluated at the moment the command is invoked.
+So if you come back a month later and run the same command, you may find that
+the packages have been updated and that could be problematic.
+
+In the next section we will create a new flake that defines the packages of the
+shell and also records the exact version of the packages that we used at the
+evaluation time for future use.
+
+In the [next chapter](../ch2) we will see how to create a permanent shell that
+will retain the same packages even if they are upgraded in the cluster, until we
+decide to upgrade them.
diff --git a/web/content/intro-nix/ch2.md b/web/content/intro-nix/ch2.md
new file mode 100644
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+---
+title: "Chapter 2: Your first shell"
+date: 2023-09-15
+weight: 2
+---
+
+## Creating a shell with flake.nix
+
+First, create an empty git repository where your shells will live:
+
+```txt
+hut% mkdir jungle-examples
+hut% cd jungle-examples
+hut% git init
+Initialized empty Git repository in /home/Computational/rarias/jungle-examples/.git/
+```
+
+And then, place a file named `flake.nix` on the repo with this content:
+
+```nix
+{
+  inputs.jungle.url = "jungle";
+  nixConfig.bash-prompt = "\[nix-develop\]$ ";
+
+  outputs = { self, jungle }:  
+  let
+    pkgs = jungle.outputs.packages.x86_64-linux;
+  in {
+    devShells.x86_64-linux.default = pkgs.mkShell rec {
+      pname = "my-shell";
+      buildInputs = with pkgs.bsc; [
+        ovni osumb # other packages here...
+      ];
+    };
+  };
+}
+```
+
+This file defines a how to create a shell in the Nix language with the
+`pkgs.mkShell` function using the packages listed in `buildInputs`. It also
+requests the packages to be taken from the *jungle* input, which corresponds to
+the set of packages that [we defined earlier](../ch1#where-packages-come-from),
+tuned for the cluster. We will describe it in more detail later.
+
+The tool `nix develop` tries to find a flake.nix in the current directory and
+enter the shell described by `devShells.x86_64-linux.default` (or the
+corresponding architecture).
+
+Now, **it is important that all the files of the repository are committed in
+git**, as nix will only read what is in the index of git. If we try to enter the
+shell with the `nix develop` command, it will complain and fail:
+
+```txt
+hut% nix develop
+warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
+error: getting status of '/nix/store/0ccnxa25whszw7mgbgyzdm4nqc0zwnm8-source/flake.nix': No such file or directory
+```
+
+The first warning states that the git directory has modified files not added to
+the index. Then the error occurs because the flake.nix is not in the index of
+git, so `nix develop` doesn't see it. So let's add it to a commit and try again:
+
+```txt
+hut% git add flake.nix
+hut% git commit flake.nix -m 'First shell'
+[master (root-commit) eb8a4ac] First shell
+ 1 file changed, 13 insertions(+)
+ create mode 100644 flake.nix
+hut% nix develop
+warning: creating lock file '/home/Computational/rarias/jungle-examples/flake.lock'
+warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
+[nix-develop]$
+```
+
+In the `flake.nix` we have set the shell prompt to `[nix-develop]` so we can
+easily spot that we are inside a `nix develop` shell. To exit:
+
+```txt
+[nix-develop]$ exit
+hut%
+```
+
+## Using the flake.lock file
+
+Now we see the `creating lock file` message and the git tree becomes dirty
+again (however, we enter the shell successfully).
+
+This `flake.lock` file that has been created collects the current state of the
+jungle packages in a file, so future invocations will use the same versions. We
+can see more details with `nix flake metadata`:
+
+```txt
+hut% nix flake metadata
+warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
+Resolved URL:  git+file:///home/Computational/rarias/jungle-examples
+Locked URL:    git+file:///home/Computational/rarias/jungle-examples
+Path:          /nix/store/bckxqjkkv52hy4pzgb96r7fchhmvmql8-source
+Revision:      eb8a4ac544a74e3995d859c751e9ff4339de6509-dirty
+Last modified: 2023-09-15 13:06:12
+Inputs:
+└───jungle: path:/nix/store/3wv6q0f3pkgw840nnkn4jsp9xi650dyj-source?lastModified=1694772033&narHash=sha256-7a09O0Jb8WncxeB32ywmQEMqJdEFLrOG/XVT9bdII6I%3D&rev=653d411b9e46076a7878be9574ed6b3bd627cff1&revCount=195
+    ├───agenix: github:ryantm/agenix/d8c973fd228949736dedf61b7f8cc1ece3236792
+    │   ├───darwin: github:lnl7/nix-darwin/87b9d090ad39b25b2400029c64825fc2a8868943
+    │   │   └───nixpkgs follows input 'jungle/agenix/nixpkgs'
+    │   ├───home-manager: github:nix-community/home-manager/32d3e39c491e2f91152c84f8ad8b003420eab0a1
+    │   │   └───nixpkgs follows input 'jungle/agenix/nixpkgs'
+    │   └───nixpkgs follows input 'jungle/nixpkgs'
+    ├───bscpkgs: git+https://pm.bsc.es/gitlab/rarias/bscpkgs.git?ref=refs/heads/master&rev=3a4062ac04be6263c64a481420d8e768c2521b80
+    │   └───nixpkgs follows input 'jungle/nixpkgs'
+    └───nixpkgs: github:NixOS/nixpkgs/e56990880811a451abd32515698c712788be5720
+```
+
+Now, as long as we keep these two files `flake.nix` and `flake.lock`, we can
+reproduce the same shell in the future, so let's add the lock file into git too.
+
+```txt
+hut% git commit -m 'Add flake.lock file'
+[master d3725ec] Add flake.lock file
+ 1 file changed, 135 insertions(+)
+ create mode 100644 flake.lock
+hut% git status
+On branch master
+nothing to commit, working tree clean
+```
+
+## Using the shell with nix develop
+
+Now, the invocations of `nix develop` won't complain that the git tree is dirty
+anymore and will enter the shell:
+
+```txt
+hut% nix develop
+[nix-develop]$
+```
+
+And the requested packages are now available:
+
+```txt
+[nix-develop]$ which ovniemu
+/nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0/bin/ovniemu
+```
+
+The packages of the shell are listed in the `$buildInputs` variable, in case you
+need to examine them:
+
+```txt
+[nix-develop]$ printf '%s\n' $buildInputs
+/nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0
+/nix/store/lnjirzllhjn2fadlqzrz7a547iawl8jc-osu-micro-benchmarks-7.1-1
+[nix-develop]$ exit
+hut%
+```
+
+In the [next chapter](../ch3) we will see how to add more packages and also how to modify
+them.
diff --git a/web/content/intro-nix/ch3.md b/web/content/intro-nix/ch3.md
new file mode 100644
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+++ b/web/content/intro-nix/ch3.md
@@ -0,0 +1,160 @@
+---
+title: "Chapter 3: Custom packages"
+date: 2023-09-15
+weight: 3
+---
+
+## Adding more packages
+
+So far we have define all the packages using:
+
+```nix
+pkgs.mkShell rec {
+  pname = "my-shell";
+  buildInputs = with pkgs.bsc; [
+    ovni osumb # other packages here...
+  ];
+};
+```
+
+This line specifies that all packages come from the `pkgs.bsc` set. We can add
+additional packages adding them to the list:
+
+```nix
+pkgs.mkShell rec {
+  pname = "my-shell";
+  buildInputs = with pkgs.bsc; [
+    ovni osumb sonar
+  ];
+};
+```
+
+And running `nix develop` again:
+
+```txt
+hut% nix develop
+warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
+[nix-develop]$ printf '%s\n' $buildInputs
+/nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0
+/nix/store/lnjirzllhjn2fadlqzrz7a547iawl8jc-osu-micro-benchmarks-7.1-1
+/nix/store/fjxj4xs0wblw3jyhp4vsrsfnlfwawifa-sonar-0.1.0
+```
+
+In the jungle cluster, the default MPI implementation is currently set to MPICH,
+as it can be shown with ldd:
+
+```txt
+[nix-develop]$ ldd $(which ovnisync) | grep mpi
+        libmpi.so.12 => /nix/store/nnnaly6hgylravdrmqkhpx1ndg5p79nc-mpich-4.1.2/lib/libmpi.so.12 (0x00007ffff5200000)
+```
+
+Now, what if we want to replace the MPI implementation by another one?
+
+## Modifying a package
+
+You notice that the packages we are using are coming directly from the ones
+specified in jungle. However, what if we need to modify some option at build
+time or change a dependency?
+
+The Nix language is used to describe how to build each package, and can be
+extended to create derived versions very easily.
+
+Let's focus on the `ovni` package. First, to load the definition we can use the
+`nix edit` command, which opens the definition file using the editor defined in
+`$EDITOR`:
+
+```txt
+hut% nix edit jungle#bsc.ovni
+...
+```
+
+This particular package has several inputs that can be modified directly:
+
+```txt
+{
+  stdenv
+, lib
+, cmake
+, mpi
+, fetchFromGitHub
+, useGit ? false
+, gitBranch ? "master"
+, gitUrl ? "ssh://git@bscpm03.bsc.es/rarias/ovni.git"
+, gitCommit ? "d0a47783f20f8b177a48418966dae45454193a6a"
+, enableDebug ? false
+}:
+...
+```
+
+For example, the `enableDebug` flag, currently set to false, affects how the
+build is configured:
+
+```txt
+cmakeBuildType = if (enableDebug) then "Debug" else "Release";
+```
+
+Now, to change this option we could replace `ovni` for our version:
+
+```nix
+{
+  inputs.jungle.url = "jungle";
+  nixConfig.bash-prompt = "\[nix-develop\]$ ";
+
+  outputs = { self, jungle }:  
+  let
+    pkgs = jungle.outputs.packages.x86_64-linux;
+    ovniDebug = pkgs.bsc.ovni.override { enableDebug = true; };
+  in {
+    devShells.x86_64-linux.default = pkgs.mkShell rec {
+      pname = "my-shell";
+      buildInputs = with pkgs.bsc; [
+        ovniDebug osumb sonar
+      ];
+    };
+  };
+}
+```
+
+And then, when we now enter the develop shell we can see that ovni gets build
+with the Debug option:
+
+```txt
+hut% nix develop -L
+warning: Git tree '/home/Computational/rarias/jungle-examples' is dirty
+ovni> unpacking sources
+ovni> unpacking source archive /nix/store/cz4si0vsw85r9s6dyiqr5ybngh9aympi-source
+ovni> source root is source
+ovni> patching sources
+ovni> updateAutotoolsGnuConfigScriptsPhase
+ovni> configuring
+ovni> fixing cmake files...
+ovni> cmake flags: ... -DCMAKE_BUILD_TYPE=Debug ...
+...
+[nix-develop]$ which ovniver
+/nix/store/hg0xs7fpibwjhsp9ajqfcbffsh69mrsm-ovni-1.3.0/bin/ovniver
+
+[nix-develop]$ file $(which ovniver) | fold
+/nix/store/hg0xs7fpibwjhsp9ajqfcbffsh69mrsm-ovni-1.3.0/bin/ovniver: ELF 64-bit L
+SB executable, x86-64, version 1 (SYSV), dynamically linked, interpreter /nix/st
+ore/9la894yvmmksqlapd4v16wvxpaw3rg70-glibc-2.37-8/lib/ld-linux-x86-64.so.2, for
+GNU/Linux 3.10.0, with debug_info, not stripped
+```
+
+And we see that the ovniver program is now compiled with debug symbols.
+
+However, this *only* replaces the ovni package that we specify in the shell. The
+sonar library also depends on ovni, but that package is still using the old one:
+
+```txt
+[nix-develop]$ find $buildInputs -name 'libovni.so.1'
+/nix/store/hg0xs7fpibwjhsp9ajqfcbffsh69mrsm-ovni-1.3.0/lib/libovni.so.1
+
+[nix-develop]$ find $buildInputs -name 'libsonar-mpi.so'
+/nix/store/fjxj4xs0wblw3jyhp4vsrsfnlfwawifa-sonar-0.1.0/lib/libsonar-mpi.so
+
+[nix-develop]$ ldd /nix/store/fjxj4xs0wblw3jyhp4vsrsfnlfwawifa-sonar-0.1.0/lib/libsonar-mpi.so | grep ovni
+        libovni.so.1 => /nix/store/0yzas8007x9djlpbb0pckcr1vhd0mcfy-ovni-1.3.0/lib/libovni.so.1 (0x00007ffff7f8d000)
+```
+
+In the [next chapter](../ch4) we will see how to replace packages in such a way
+that all the dependences are automatically updated too.
diff --git a/web/content/intro-nix/ch4.md b/web/content/intro-nix/ch4.md
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@@ -0,0 +1,29 @@
+---
+title: "Chapter 4: Adding an overlay"
+date: 2023-09-15
+weight: 4
+---
+
+NOTE: We shouldn't be instructing users to use an overlay to replace packages in
+`bsc.` until we have determined if we move them to the root attribute set
+first!
+
+```nix
+{
+  inputs.jungle.url = "jungle";
+  nixConfig.bash-prompt = "\[nix-develop\]$ ";
+
+  outputs = { self, jungle }:  
+  let
+    pkgs = jungle.outputs.packages.x86_64-linux;
+    ovniDebug = pkgs.bsc.ovni.override { enableDebug = true; };
+  in {
+    devShells.x86_64-linux.default = pkgs.mkShell rec {
+      pname = "my-shell";
+      buildInputs = with pkgs.bsc; [
+        ovniDebug osumb sonar
+      ];
+    };
+  };
+}
+```