Storage optimization

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A recurring problem with NixOS is lack of space on /. Even if you are using Nix only occasionally, it is easy for /nix/store to go beyond 50GiB. Here are generic notes on how to not run out of space too often.

Garbage collection

The Nix store sometimes contains entries which are no longer useful.[cf. 1] They can be deleted with nix-collect-garbage -d [cf. 2] or nix-store --gc.[cf. 3]

Note that if a result file still exists in the file system, all the dependencies used to build it will be kept. To see which result files prevent garbage collection, run:

$ 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/system-398-link -> /nix/store/wmndyzzrbc9fyjw844jmvzwgwgcinq7s-nixos-system-iron-16.0916.09pre.custom
/root/forkstat/result -> /nix/store/i5glmg3wk2a48x52rhd92zip1cmc0kq9-forkstat-git
/run/booted-system -> /nix/store/8jkrl9jyq7hqxb6xpwcaghpdm26gq98j-nixos-system-iron-16.0916.09pre.custom
/run/current-system -> /nix/store/wmndyzzrbc9fyjw844jmvzwgwgcinq7s-nixos-system-iron-16.0916.09pre.custom

Run as root

nix-collect-garbage -d operates only for the current user. To clear system profiles, run it with root privileges.

Look for result symlinks

If you use nix-build, but not --no-build-output, your FS will be filled with result symlinks to various derivations. In the example above, note the following symlinks:

/home/danbst/stack/new/website/server/result -> /nix/store/1jhmp6vl364p32r8bjigk65qh1xa562f-server-0.1.0.0
/home/ec2-user/result -> /nix/store/q35aq2sh5dbyka6g6f6qb7b8msxwds5m-nixos-system-iron-16.03.1299.a8e0739
/root/forkstat/result -> /nix/store/i5glmg3wk2a48x52rhd92zip1cmc0kq9-forkstat-git

How much space do these (apparently) abandoned derivations use?

$ du -sch $(nix-store -qR /root/forkstat/result /home/ec2-user/result /home/danbst/stack/new/website/server/result)
...
3.4G    total

Not all of the derivations are garbage in this case, but quite a few are:

# rm /root/forkstat/result /home/ec2-user/result /home/danbst/stack/new/website/server/result
# nix-collect-garbage -d
...
690 store paths deleted, 1817.99 MiB freed

Look for system derivations in particular. Those are created on many occasions, for example when running nixos-rebuild build-vm

Reboot

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

Pinning

When you invoke nix-shell with

$ nix-instantiate shell.nix --indirect --add-root $DIR/.nix-gc-roots/shell.drv ...

then you'll have a persistent environment which won't be garbage collected. It is useful when you don't want to spend time waiting for redownloads every time you enter the shell.

A little problem exists though. GC roots are numbered sequentially, so if you change shell.nix to contain less derivations, and name of last GC root will start with shell.drv-7, then shell.drv-{8,9,10,11,12}* will be dangling and unused. To overcome this problem you should remove GC roots dir periodically (or just before nix-shell)

Obviously, you should remove the GC roots directory for projects you don't plan to work on.

Automation

It is possible to enable periodic automatic GC,[cf. 4] for example like this:

nix.gc = {
  automatic = true;
  dates = "weekly";
  options = "--delete-older-than 30d";
};

This can result in redownloads (tarballs fetched with import (builtins.fetchFromTarball ...) for example are not referenced anywhere and removed on GC), but it frees you from runnning GC manually.

It is also possible to automatically run garbage collection whenever there is not enough space left.[cf. 5] For example, to free up to 1GiB whenever there is less than 100MiB left:

nix.extraOptions = ''
  min-free = ${toString (100 * 1024 * 1024)}
  max-free = ${toString (1024 * 1024 * 1024)}
'';

This is particularly useful when the store is on its own partition, see below.

Moving the store

/nix can reside on another device. This is useful if your root device is very small, and that you have another, larger drive at hand. It is easiest to set up when installing NixOS. To move /nix to another device on an existing NixOS installation:

  1. Create a new ext4 partition and mount it over /mnt
  2. rsync -a everything from /nix to /mnt
  3. bind /mnt to /nix (e.g. using mount) and rerun nixos-rebuild switch with something like
    fileSystems."/nix" = { device = "/dev/disk/by-label/nix"; options = [ "noatime" ]; };
    
  4. reboot to be sure /nix/store is properly mounted
  5. bind mount / to /old_root, and remove /old_root/nix

Keep in mind that all commands like mount and bash point to some executable in /nix/store, so never mount an empty disk over /nix or /nix/store, otherwise you will be locked out until reboot!

Optimizing the store

Run nix-store --optimise in case you really need some a bit of space. It is a lengthy operation, and after multiple-outputs PR landing, it became less useful.

# nix-store --optimise
985.41 MiB freed by hard-linking 251942 files

See also

  1. Nix Manual, 11. Garbage Collection
  2. nix-collect-garbage(1)
  3. nix-store(1) , under OPERATION --GC
  4. nix.gc
  5. min-free and max-free