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A recurring problem with NixOS is lack of space on <code>/</code>. Even if you only ocasionally use Nix, it is easy for <code>/nix/store</code> to grow beyond 50GiB. Here are generic notes on how to not run out of space too often.
{{Merge|Nix store}}


== Optimizing the store ==
A recurring problem with NixOS is lack of space on <code>/</code>. Even if you only occasionally use Nix, it is easy for <code>/nix/store</code> to grow beyond reasonable sizes. What follows are generic notes on how to reduce the growth of the [[Nix store]].
The option and command below save space by hardlinking store files:


==== Automatically ====
== Optimising the store ==
<syntaxhighlight lang="nix">
Here, we demonstrate how to configure <code>nix</code> to save space via hardlinking store files.
nix.autoOptimiseStore = true;
 
</syntaxhighlight>
=== Automatic ===
To turn on periodic optimisation of the nix store, set the following option in <code>/etc/nixos/configuration.nix</code>:
{{file|configuration.nix|nix|<nowiki>
nix.optimise.automatic = true;
nix.optimise.dates = [ "03:45" ]; # Optional; allows customizing optimisation schedule
</nowiki>}}
 
Alternatively, the store can be optimised during ''every'' build. This may slow down builds, as discussed [https://github.com/NixOS/nix/issues/6033 here]. To enable this behavior, set the following option:
{{file|configuration.nix|nix|<nowiki>
nix.settings.auto-optimise-store = true;
</nowiki>}}


But this option only applies to new files: you'll still need to manually optimise your store once, after you enable this option.
{{Tip|This option only applies to new files, so we recommend manually optimising your nix store when first setting this option.}}


==== Manually ====
=== Manual ===
Run <code>nix-store --optimise</code>. This will take some time to complete.
Run  
{{ic|# nix-store --optimise}}.  
This is a potentially long operation.


== Garbage collection ==
== Garbage collection ==


The Nix store sometimes contains entries which are no longer useful.<ref group="cf.">[https://nixos.org/nix/manual/#sec-garbage-collection Nix Manual, 11. Garbage Collection]</ref> They can be deleted with {{ic|nix-collect-garbage -d}} <ref group="cf.">{{man|nix-collect-garbage|sec=1}}</ref> or {{ic|nix-store --gc}}.<ref group="cf.">{{man|nix-store|sec=1}}, under {{ic|OPERATION --GC}}</ref>
By default, the Nix store will not remove any entries that are no longer used, thus it can accumulate derivations you might no longer need.<ref group="cf.">[https://nixos.org/nix/manual/#sec-garbage-collection Nix Manual, 11. Garbage Collection]</ref> They can be deleted with {{ic|nix-collect-garbage}} <ref group="cf.">{{man|nix-collect-garbage|sec=1}}</ref> or {{ic|nix-store --gc}}.<ref group="cf.">{{man|nix-store|sec=1}}, under {{ic|OPERATION --GC}}</ref>
 
=== Removing old generations ===
NixOS keeps old configurations of your system around so that you can always rollback to a previous configuration if something goes wrong. You can also select which generation to boot into via GRUB. However these previous generations are [[Storage optimization#Garbage collection roots|GC roots]] (see below) that can keep around old, unnecessary software in your nix store. You can check what system generations you have by:<syntaxhighlight lang="console">$ sudo nix-env -p /nix/var/nix/profiles/system --list-generations
...
  58  2021-09-04 02:56:54
  59  2021-09-05 07:12:43
  60  2021-09-05 22:12:13  (current)</syntaxhighlight>
 
You can remove all other NixOS generations besides the current one:
<syntaxhighlight lang="console">
$ sudo nix-collect-garbage -d
...
4394 store paths deleted, 3467.28 MiB freed
</syntaxhighlight>
 
There are also user-specific generations for different things (eg. home-manager). These can be removed with:
<syntaxhighlight lang="console">
$ nix-collect-garbage -d
</syntaxhighlight>
 
=== Garbage collection roots ===


Note that if a result file still exists in the file system, and your Nix configuration has both <code>keep-outputs = true</code> and <code>keep-derivations = true</code>, all the dependencies used to build it will be kept. To see which result files prevent garbage collection, run:
Note that if a result file still exists in the file system, and your Nix configuration has both <code>keep-outputs = true</code> and <code>keep-derivations = true</code>, all the dependencies used to build it will be kept. To see which result files prevent garbage collection, run:
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<syntaxhighlight lang="console">
<syntaxhighlight lang="console">
$ nix-store --gc --print-roots
$ nix-store --gc --print-roots
/home/danbst/dev/test-shell/.shell.drv -> /nix/store/4diqwczyjipdqyi7aj34wfagblbhfjr9-nixops-1.4
/home/danbst/dev/test-shell/.shell.drv-2 -> /nix/store/62h3c4d6rdnlxichixqg8h9jxi8nhxk0-stdenv
/home/danbst/dev/test-shell/.shell.drv-2-doc -> /nix/store/14gnv1q1w0n9qwa3q23idsqvn51354y8-bash-4.3-p42-doc
/home/danbst/stack/new/website/server/result -> /nix/store/1jhmp6vl364p32r8bjigk65qh1xa562f-server-0.1.0.0
/home/danbst/testing/.nix-gc-roots/shell.drv -> /nix/store/v3vqf48awjjzjivrx15kfqdh1d7cg4mq-sshpass-1.05
...
/home/danbst/testing/.nix-gc-roots/shell.drv-12 -> /nix/store/a2li4sl9pxh9aflqia2gp7w88ayvjwci-bash-4.3-p42
/home/danbst/testing/.nix-gc-roots/shell.drv-12-doc -> /nix/store/kcswyb1d8zimkym0pjfi2fj1dly1w34w-bash-4.3-p42-doc
/home/danbst/testing/.nix-gc-roots/shell.drv-12-info -> /nix/store/njb817fwiafswzwvj9skw7w7k6b3fnbi-bash-4.3-p42-info
/home/ec2-user/result -> /nix/store/q35aq2sh5dbyka6g6f6qb7b8msxwds5m-nixos-system-iron-16.03.1299.a8e0739
/nix/var/nix/profiles/per-container/analyt/system-3-link -> /nix/store/snrj72189wh9va23fawl3v80v92xnxlm-nixos-system-iron-16.03.1291.efe2d64
/nix/var/nix/profiles/per-container/d-live/system-6-link -> /nix/store/cp2c58hnczsjk5h69ksajq5xfhsyhl6v-nixos-system-iron-16.03.1299.a8e0739
/nix/var/nix/profiles/per-container/d-test/system-4-link -> /nix/store/n1w7ywjg65x8iimchznxcyygbgmyfh55-nixos-system-iron-16.03.1287.6ac7ffd
/nix/var/nix/profiles/per-container/dashboard/system-41-link -> /nix/store/7qk19pkwgq0h3a1q9dcql3nks40rr75s-nixos-system-iron-16.03.1340.5a090dd
...
...
/nix/var/nix/profiles/per-container/ttt/system-1-link -> /nix/store/1kj9qs5gl3421jlkl3jfc2kqdsl8akwr-nixos-system-ttt-16.03.977.1da05df
/nix/var/nix/profiles/per-user/danbst/channels-1-link -> /nix/store/s0qay9qyqrn92zayldbvvj3zrfcl7a72-user-environment
/nix/var/nix/profiles/per-user/danbst/profile-28-link -> /nix/store/69ds606146dqml04sm0fbpqwnv2w8i3q-user-environment
/nix/var/nix/profiles/per-user/ec2-user/profile-7-link -> /nix/store/y2hc7zsnkzys9ba6xaijvjhff03rcgpy-user-environment
/nix/var/nix/profiles/per-user/root/channels-4-link -> /nix/store/254b6pkhhnjywvj5c0lp2vdai8nz4p0g-user-environment
/nix/var/nix/profiles/per-user/root/channels-4-link -> /nix/store/254b6pkhhnjywvj5c0lp2vdai8nz4p0g-user-environment
/nix/var/nix/profiles/system-398-link -> /nix/store/wmndyzzrbc9fyjw844jmvzwgwgcinq7s-nixos-system-iron-16.0916.09pre.custom
/nix/var/nix/profiles/system-398-link -> /nix/store/wmndyzzrbc9fyjw844jmvzwgwgcinq7s-nixos-system-iron-16.0916.09pre.custom
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GC roots can be found in <code>/nix/var/nix/gcroots</code>. The following script demonstrates how this directory can be used to (for example) query the state of manually made result symlinks:
GC roots can be found in <code>/nix/var/nix/gcroots</code>. The following script demonstrates how this directory can be used to (for example) query the state of manually made result symlinks:


<syntaxhighlight lang="bash">
<syntaxhighlight lang="console">
find -H /nix/var/nix/gcroots/auto -type l | xargs -I {} sh -c 'readlink {}; realpath {}; echo'
$ find -H /nix/var/nix/gcroots/auto -type l | xargs -I {} sh -c 'readlink {}; realpath {}; echo'
</syntaxhighlight>
</syntaxhighlight>


This acts a simpler (but faster) version of <code>--print-roots</code> and could be implemented as a bash alias for convenience.
This acts a simpler (but faster) version of <code>--print-roots</code> and could be implemented as a bash alias for convenience.


=== Run as root ===
{{Tip|
<code>nix-collect-garbage -d</code> operates only for the current user. To clear system profiles, run it with root privileges.
<code>nix-collect-garbage -d</code> operates only for the current user. To clear system profiles, run it with root privileges.
}}


=== Look for <code>result</code> symlinks ===
=== Look for <code>result</code> symlinks ===
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=== Reboot ===
=== Reboot ===
As you see, the reference in <code>/run/booted-system</code> is a GC root, so it won't be cleared until reboot. If you don't want to reboot, just <code>rm /run/booted-system</code> that link and rerun <code>sudo nix-collect-garbage</code>.
As you see, the reference in <code>/run/booted-system</code> is a GC root, so it won't be cleared until reboot. If you don't want to reboot, just <code>rm /run/booted-system</code> that link and rerun <code>sudo nix-collect-garbage</code>.


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=== Automation ===
=== Automation ===


It is possible to enable periodic automatic GC,<ref group="cf.">{{nixos:option|nix.gc}}</ref> for example like this:
Garbage collection can be automated,<ref group="cf.">{{nixos:option|nix.gc}}</ref> for example:


<syntaxhighlight lang="nix">
{{file|configuration.nix|nix|<nowiki>
nix.gc = {
nix.gc = {
   automatic = true;
   automatic = true;
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   options = "--delete-older-than 30d";
   options = "--delete-older-than 30d";
};
};
</syntaxhighlight>
</nowiki>}}


If using nix-darwin, use this to run on 0th day of every week:
{{file|configuration.nix|nix|<nowiki>
nix.gc = {
  automatic = true;
  interval = { Weekday = 0; Hour = 0; Minute = 0; };
  options = "--delete-older-than 30d";
};
</nowiki>}}
This can result in redownloads (tarballs fetched with <code>import (builtins.fetchTarball ...)</code> for example are not referenced anywhere and removed on GC), but it frees you from runnning GC manually.
This can result in redownloads (tarballs fetched with <code>import (builtins.fetchTarball ...)</code> for example are not referenced anywhere and removed on GC), but it frees you from runnning GC manually.


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== Moving the store ==
== Moving the store ==


{{ic|/nix}} can reside on another device. This is useful if your root device is very small, and you have another, larger drive available.
{{ic|/nix}} can reside on another device, which is useful if your root device is very small, and you have another, larger drive available.


If the second mountpoint is on the same device, some benefit can still be gained by formatting the partition it points to with a different file system. For example: on a Raspberry Pi, f2fs could possibly be used for a gain in I/O throughput.
If the new partition is on the same device, some benefit can be gained by formatting the partition on which {{ic|nix}} resides with a different file system. For example: on a Raspberry Pi, {{ic|f2fs}} could be used for a gain in I/O throughput.


Regardless of <code>/nix</code>'s filesystem, it can also be mounted with <code>noatime</code> (as seen in the example below). This will reduce metadata writes, improving I/O and the device's lifespan.
Regardless of <code>/nix</code>'s filesystem, it can also be mounted with <code>noatime</code> (as seen in the example below). This will reduce metadata writes, improving I/O and the device's lifespan.
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This is easiest to set up while installing NixOS, but <code>/nix</code> can be moved on a live system:
This is easiest to set up while installing NixOS, but <code>/nix</code> can be moved on a live system:


# Create a new partition and mount it over <code>/mnt</code>
''All commands below are executed with root privileges''
# <code>rsync -aAxv</code> everything from <code>/nix</code> to <code>/mnt</code>
 
# Bind <code>/mnt</code> to <code>/nix</code> (e.g. using <code>mount</code>).
# Create a new partition
# Restart nix daemon with <code>systemctl restart nix-daemon.service </code> <code>systemctl restart nix-daemon.socket </code>.
# Mount this new partition over <code>/mnt</code> <syntaxhighlight lang="console">
# Rerun <code>nixos-rebuild switch</code> with something like <syntaxhighlight lang="bash">fileSystems."/nix" = { device = "/dev/disk/by-label/nix"; neededForBoot = true; options = [ "noatime" ]; };</syntaxhighlight>
# mount -o defaults,noatime /dev/disk/by-label/nix /mnt/nix
</syntaxhighlight>
# Copy everything from <code>/nix</code> to <code>/mnt</code> ''Trailing slashes are important'', in that without them, <code>rsync</code> will create an additional directory of the same name at the destination. <syntaxhighlight lang="console">
# rsync --archive --hard-links --acls --one-file-system --verbose /nix/{store,var} /mnt/nix
</syntaxhighlight>
# Mount the new partition as the new <code>/nix</code> <syntaxhighlight lang="console">
# umount /mnt/nix
# mount /dev/disk/by-label/nix /nix
</syntaxhighlight>
# Restart {{ic|nix-daemon}} <syntaxhighlight lang="console">
$ systemctl stop nix-daemon.service
$ systemctl restart nix-daemon.socket
$ systemctl start nix-daemon.service
</syntaxhighlight>
# Add the new <code>/nix</code> partition to your <code>/etc/nixos/configuration.nix</code> <syntaxhighlight lang="nix">
{
  # ...
  fileSystems."/nix" = {
    device = "/dev/disk/by-label/nix";
    fsType = "ext4";
    neededForBoot = true;
    options = [ "noatime" ];
  };
}
</syntaxhighlight>
# Apply your configuration <syntaxhighlight lang="console">
# nixos-rebuild switch
</syntaxhighlight>
# Reboot to be sure <code>/nix/store</code> is properly mounted


# Reboot to be sure <code>/nix/store</code> is properly mounted.
''Optionally''
# Once you are sure everything works, you can delete the old store doing a bind mount <code>/</code> to <code>/old_root</code>, and remove <code>/old_root/nix</code>.
# After reboot, check that <code>/nix</code> is mounted over your partition <syntaxhighlight lang="console">
# mount | grep "/nix" && echo "Nix store is on a new partition" || echo "Nix is on the old partition"
</syntaxhighlight>
# Once '''you are sure''' everything works, you can delete the old store <syntaxhighlight lang="console">
# mkdir /tmp/old_root
# mount --bind / /tmp/old_root
# rm --recursive /tmp/old_root/nix
# umount /tmp/old_root
# rmdir /tmp/old_root
</syntaxhighlight>


Keep in mind that all commands like <code>mount</code> and <code>bash</code> point to some executable in <code>/nix/store</code>, so never mount an empty disk over <code>/nix</code> or <code>/nix/store</code>, otherwise you will be locked out until reboot!
{{Tip|Keep in mind that all commands like <code>mount</code> and <code>bash</code> point to some executable in <code>/nix/store</code>. It is possible to get locked out of a system if one mistakenly mounted an empty drive to {{ic|nix}}. }}


== See also ==
== See also ==


<references group="cf."/>
<references group="cf."/>
[[Category:NixOS]]
[[Category:Nix]]
Retrieved from "https://wiki.nixos.org/wiki/Storage_optimization"