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Storage optimization: Difference between revisions

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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.
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 reasonable sizes. What follows are generic notes on how to reduce the growth of the {{nix:store}}.  


== Optimizing the store ==
== Optimising the store ==
The option and command below save space by hardlinking store files:
Here we demonstrate how to configure <code>nix</code> to save space via hardlinking store files.


==== Automatically ====
==== Automatic ====
<syntaxhighlight lang="nix">
To turn on automatic optimisation for newer derivations, add the following option to <code>/etc/nixos/configuration.nix</code>:
{{file|configuration.nix|nix|<nowiki>
nix.autoOptimiseStore = true;
nix.autoOptimiseStore = true;
</syntaxhighlight>
</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 in addition to 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>
The Nix store accumulates 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>


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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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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=== 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>}}


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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# Create a new partition
# Create a new partition
# Mount this new partition over <code>/mnt</code> <syntaxhighlight lang="bash">
# Mount this new partition over <code>/mnt</code> <syntaxhighlight lang="console">
mount /dev/disk/by-label/nix /mnt
# mount /dev/disk/by-label/nix /mnt
</syntaxhighlight>
</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="bash">
# 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 --acls --one-file-system --verbose /nix/store/ /mnt/store
# rsync --archive --acls --one-file-system --verbose /nix/store/ /mnt/store
rsync --archive --acls --one-file-system --verbose /nix/var/ /mnt/var
# rsync --archive --acls --one-file-system --verbose /nix/var/ /mnt/var
</syntaxhighlight>
</syntaxhighlight>
# Use the new partition as new <code>/nix</code> <syntaxhighlight lang="bash">
# Mount the new partition as the new <code>/nix</code> <syntaxhighlight lang="console">
umount /mnt
# umount /mnt
mount /dev/disk/by-label/nix /nix
# mount /dev/disk/by-label/nix /nix
</syntaxhighlight>
</syntaxhighlight>
# Restart nix daemon <syntaxhighlight lang="bash">
# Restart {{ic|nix-daemon}} <syntaxhighlight lang="console">
systemctl stop nix-daemon.service
$ systemctl stop nix-daemon.service
systemctl restart nix-daemon.socket
$ systemctl restart nix-daemon.socket
systemctl start nix-daemon.service
$ systemctl start nix-daemon.service
</syntaxhighlight>
</syntaxhighlight>
# Add mounting the <code>/nix</code> partition to your <code>/etc/nixos/configuration.nix</code> <syntaxhighlight lang="nix">
# Add the new <code>/nix</code> partition to your <code>/etc/nixos/configuration.nix</code> <syntaxhighlight lang="nix">
{
{
   # ...
   # ...
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     options = [ "noatime" ];
     options = [ "noatime" ];
   };
   };
  # ...
}
}
</syntaxhighlight>
</syntaxhighlight>
# Apply your configuration <syntaxhighlight lang="bash">
# Apply your configuration <syntaxhighlight lang="console">
nixos-rebuild switch
# nixos-rebuild switch
</syntaxhighlight>
</syntaxhighlight>
# Reboot to be sure <code>/nix/store</code> is properly mounted
# Reboot to be sure <code>/nix/store</code> is properly mounted


''Optionally''
''Optionally''
# After reboot, check that <code>/nix</code> is mounted over your partition <syntaxhighlight lang="bash">
# After reboot, check that <code>/nix</code> is mounted over your partition <syntaxhighlight lang="console">
mount | grep "/nix" && echo "Nix seems to use your new partition" || echo "It seems that something bad happened"
# mount | grep "/nix" && echo "Nix store is on a new partition" || echo "Nix is on the old partition"
</syntaxhighlight>
</syntaxhighlight>
# Once '''you are sure''' everything works, you can delete the old store <syntaxhighlight lang="bash">
# Once '''you are sure''' everything works, you can delete the old store <syntaxhighlight lang="console">
mkdir /tmp/old_root
# mkdir /tmp/old_root
mount --bind / /tmp/old_root
# mount --bind / /tmp/old_root
rm --recursive /tmp/old_root/nix
# rm --recursive /tmp/old_root/nix
umount /tmp/old_root
# umount /tmp/old_root
rmdir /tmp/old_root
# rmdir /tmp/old_root
</syntaxhighlight>
</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."/>
Retrieved from "https://wiki.nixos.org/wiki/Storage_optimization"