# How to create a references free NixOS image?
**URL:** https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642
**Category:** Help
**Created:** [May 23, 2025, 8:36am UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642 "2025-05-23T08:36:16Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@basvandijk](https://discourse.nixos.org/u/basvandijk)
#### Post date: [May 23, 2025, 8:36am UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642/1 "2025-05-23T08:36:17Z")
The flake in [GitHub - basvandijk/ref-free-img: How to create a references free NixOS image?](https://github.com/basvandijk/ref-free-img) defines a simple NixOS disk image that can be build with:
```shell
$ nix build
$ tree result
result
├── nix-support
│ └── hydra-build-products
└── nixos.img.zst
```
I would like to `nix copy` the result to another system.
The reason for `nix copy` is that I’m building the image on [https://nixbuild.net](https://nixbuild.net) and I would like to get the image to my local machine / GitHub runner so I’ll need to use the method described in:
[Running Remote Builds - nixbuild.net documentation](https://docs.nixbuild.net/remote-builds/#retrieving-build-output-from-remote-stores)
However when copying I see that a lot of other /nix/store paths are copied as well which means this image has run-time dependencies on these other paths. Indeed:
```shell
$ nix-store -q --references result
/nix/store/045cq354ckg28php9gf0267sa4qgywj9-X-Restart-Triggers-systemd-timesyncd
/nix/store/0597v01rkmjdqn52idddi5x2vp08l847-perl5.40.0-List-Compare-0.55
/nix/store/0i7mzq93m8p7253bxnh7ydahmjsjrabk-gzip-1.14-man
/nix/store/0ip389clsbrbjmhmrysgfghqnhx8qlfd-glibc-locales-2.40-66
/nix/store/15k9rkd7sqzwliiax8zqmbk9sxbliqmd-X-Restart-Triggers-systemd-journald-
/nix/store/184bcjcc97x3klsz63fy29ghznrzkipg-zstd-1.5.7-man
/nix/store/cg9s562sa33k78m63njfn1rw47dp9z0i-glibc-2.40-66
...
/nix/store/x4a9ksmwqbhirjxn82cddvnhqlxfgw8l-linux-headers-static-6.12.7
/nix/store/x4b392vjjza0kz7wxbhpji3fi8v9hr86-gtest-1.16.0
/nix/store/xv0pc5nc41v5vi0lac1i2d353s3rqlkm-libxml2-2.13.8
/store/y180fqjr06vrg3fn09bx7dsxz7vvnzqk-etc
/nix/store/y7y1v7l88mxkljbijs7nwzm1gcg9yrjw-extra-utils
/nix/store/za3c1slqlz1gpm6ygzwnh3hd2f0lg31z-libblake3-1.8.2
/nix/store/lxdb1wc09p0494x4gslhk0hxn1q9l83s-nixos-disk-image
```
Nix probably tracks these as run-time dependencies since some, but not all, /nix/store paths are contained in the zstandard compressed result:
```shell
$ strings result/nixos.img.zst | grep /nix/store
/nix/store/y7y1v7l88mxkljbijs7nwzm1gcg9yrjw-utils/bin
/nix/store/m4qaar099vcj0dgq4xdvhlbc8z4v9m22-getty/bin/
CLINKTYPE: dynamicVERSION: 1.12EXE_: >
export _ARCHIVE=/nix/store/0ip389clsbrbjmhmrysgfghqnhx8qlfd-2.40-66/lib/-archive60da146zpfdi0iplbg4hzpirb30vb5g7-perl5.40.0-XML-SAX-Base-1.09/site_perl//XML/SAX/.pm
/nix/store/1q9lw4r2mbap8rsr8cja46nap6wvrw2p-bash-active-5.2p37sh
libdir='/nix/store/2bjcjfzxnwk3zjhkrxi3m762p8dv6f1s-libcap-ng-0.8.5/lib'unix.so likeauth nullok try_first_pass # unix (order 11600)
prefix="/nix/store/za3c1slqlz1gpm6ygzwnh3hd2f0lg31z--1.8.2"
/nix/store/al9x8cr5xifp3qd2f5cdzh6z603kb5ps-perl-5.40.0/lib/perl5/x86_64-linux--multi/autCOREIPC
/nix/store/vxmnihhgnkyd2yh1y6gsyrw7lzqyh0sn-perl5.40.0-File-Slurp-9999.32/lib//site_perl///.pm
#!/nix/store/xy4jjgw87sbgwylm5kn047d9gkbhsr9x-bash-5.2p37/bin/sh
```
Since the `nixos.img.zst` is a standalone image, i.e. it contains its own /nix/store, it shouldn’t need to depend on anything in my /nix/store.
**Is there a way to “erase” these dependenies such that a `nix copy` only has to copy the final image and not all those dependencies?**
---
### Author:  [@pwaller](https://discourse.nixos.org/u/pwaller)
#### Post date: [May 23, 2025, 8:57am UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642/2 "2025-05-23T08:57:37Z")
You want to specify something like:
```auto
__structuredAttrs = true;
unsafeDiscardReferences.out = true;
```
in the derivation for your image.
[https://nix.dev/manual/nix/2.28/nix-2.28.html#other-output-modifications](https://nix.dev/manual/nix/2.28/nix-2.28.html#other-output-modifications)
---
### Author:  [@basvandijk](https://discourse.nixos.org/u/basvandijk)
#### Post date: [May 23, 2025, 10:12am UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642/3 "2025-05-23T10:12:56Z")
Thanks @pwaller, that seems exactly what I need!
Unfortunately when I [try to override](https://github.com/basvandijk/ref-free-img/compare/unsafeDiscardReferences) the [nixos/lib/make-disk-image.nix](https://github.com/NixOS/nixpkgs/blob/bc231d40cc1c0294daba2917ccd950cdcf4f514b/nixos/lib/make-disk-image.nix) derivation I run into:
```shell
$ nix build
error: attribute 'overrideAttrs' missing
at /nix/store/aqjpb4q7g3birbrx47y6c9cmdlhs0d1i-source/flake.nix:32:13:
31|
32| img = nixos.diskImage.overrideAttrs (_oldAttrs: {
| ^
33| __structuredAttrs = true;
```
I suspect this is because `make-disk-image.nix` [calls](https://github.com/NixOS/nixpkgs/blob/bc231d40cc1c0294daba2917ccd950cdcf4f514b/nixos/lib/make-disk-image.nix#L636) `pkgs.vmTools.runInLinuxVM` which in turn [overrides the derivation using overrideDerivation](https://github.com/NixOS/nixpkgs/blob/e396b3f7d4807d5cbc66d79fb224e1d2e0beb418/pkgs/build-support/vm/default.nix#L374).
Maybe that makes the resulting derivation not overridable anymore?
---
### Author:  [@basvandijk](https://discourse.nixos.org/u/basvandijk)
#### Post date: [May 23, 2025, 10:18am UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642/4 "2025-05-23T10:18:48Z")
Oh but [using](https://github.com/basvandijk/ref-free-img/compare/unsafeDiscardReferences-overrideDerivation) `pkgs.lib.overrideDerivation` works:
```shell
$ nix build
$ nix-store -q --references result
# empty :)
```
Thank you!
---
### Author:  [@basvandijk](https://discourse.nixos.org/u/basvandijk)
#### Post date: [May 23, 2025, 10:36am UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642/5 "2025-05-23T10:36:16Z")
This does beg the question:
Shouldn’t [nixos/lib/make-disk-image.nix](https://github.com/NixOS/nixpkgs/blob/bc231d40cc1c0294daba2917ccd950cdcf4f514b/nixos/lib/make-disk-image.nix) set `unsafeDiscardReferences.out = true;` itself?
---
### Author:  [@adamcstephens](https://discourse.nixos.org/u/adamcstephens)
#### Post date: [May 23, 2025, 12:01pm UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642/6 "2025-05-23T12:01:00Z")
Seems like there should at least be an option to easily enable.
---
### Author:  [@basvandijk](https://discourse.nixos.org/u/basvandijk)
#### Post date: [May 25, 2025, 12:38pm UTC](https://discourse.nixos.org/t/how-to-create-a-references-free-nixos-image/64642/7 "2025-05-25T12:38:03Z")
It turns out that in my production code overriding the image derivation with `__structuredAttrs` like:
```nix
pkgs.lib.overrideDerivation nixos.diskImage (_oldAttrs: {
__structuredAttrs = true;
unsafeDiscardReferences.out = true;
});
```
results in the error:
```shell
mkdir: missing operand
```
So I guess [nixos/lib/make-disk-image.nix](https://github.com/NixOS/nixpkgs/blob/2c0ff14313957630567265eb86c63af18416b275/nixos/lib/make-disk-image.nix) is not fully compatible with `__structuredAttrs`.
So what I’m going for now is just duplicating the derivation using a function like:
```nix
discardReferencesInOut = img:
pkgs.runCommand (img.name + "-out-refs-discarded") {
__structuredAttrs = true;
unsafeDiscardReferences.out = true;
} "cp -r ${img} $out";
```
This will result in the image being in the /nix/store twice but in my case I only have to fetch the final references free image from [nixbuild.net](http://nixbuild.net).