Garbage Collection

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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 50G.

Here are generic notes on how to not run out of space too often. I assume you already know about nix-collect-garbage and do care about used disk space.

Split /nix to a separate LVM volume

If your root device is fixed and small-sized, you should move /nix to a separate device. For example, you can run it on LVM. Optimally, you would set this up when installing NixOS, otherwise you might have to go through these steps:

  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!

That's it! You moved /nix/store!

Enable periodic Garbage Collection

Next you may enable periodic auto GC, for example like this:

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

This can result into some redownloads (if you ever use import (builtins.fetchFromTarball ...) all these fetched tarballs are not referenced anywhere and removed on GC), but overall it frees you from runnning GC manually often.

You will never be free of manual GC!

One of least mentioned problems with nix-collect-garbage -d is it doesn't collect all the garbage! The key to understand how's that, is to run this command:

$ 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

Don't forget to run nix-collect-garbage -d as root

Run nix-collect-garbage -d with sudo or su, or logged in as root, otherwise you are clearing only your old user profiles, not system-wide profiles.

Look for result symlinks

If you use nix-build, but don't use --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

Especially look for system derivations. Those are created in many cases, for example, when running nixos-rebuild build-vm

Don't forget to 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.

Proper nix-shell GC 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.

Last resort - --optimise

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