Chapter 11. Garbage Collector
Welcome to the 11th Nix pill. In the previous 10th pill we managed to obtain a self-contained development environment for a project. The concept is that nix-build is able to build a derivation in isolation, while nix-shell is able to drop us in a shell with (almost) the same environment used by nix-build. This allows us to debug, modify and manually build software.
Today we stop packaging and look at a mandatory nix component, the garbage collector. When using nix tools, often derivations are built. This include both .drv files and out paths. These artifacts go in the nix store, but we've never cared about deleting them until now.
11.1. How does it work
Other package managers, like dpkg, have ways of removing unused software. Nix is much more precise in its garbage collection compared to these other systems.
I bet with dpkg, rpm or similar traditional packaging systems, you end up having some unnecessary packages installed or dangling files. With nix this does not happen.
How do we determine whether a store path is still needed? The same way programming languages with a garbage collector decide whether an object is still alive.
Programming languages with a garbage collector have an important concept in order to keep track of live objects: GC roots. A GC root is an object that is always alive (unless explicitly removed as GC root). All objects recursively referred to by a GC root are live.
Therefore, the garbage collection process starts from GC roots, and recursively mark referenced objects as live. All other objects can be collected and deleted.
In Nix there's this same concept. Instead of being objects, of course,
GC roots are store paths.
The implementation is very simple and transparent to the user. GC roots
are stored under
/nix/var/nix/gcroots. If there's a
symlink to a store path, then that store path is a GC root.
Nix allows this directory to have subdirectories: it will simply recurse directories in search of symlinks to store paths.
So we have a list of GC roots. At this point, deleting dead store paths is as easy as you can imagine. We have the list of all live store paths, hence the rest of the store paths are dead.
In particular, Nix first moves dead store paths to
/nix/store/trash which is an atomic operation.
Afterwards, the trash is emptied.
11.2. Playing with the GC
Before playing with the GC, first run the nix garbage collector once, so that we have a clean playground for our experiments:
$ nix-collect-garbage finding garbage collector roots... [...] deleting unused links... note: currently hard linking saves -0.00 MiB 1169 store paths deleted, 228.43 MiB freed
Perfect, if you run it again it won't find anything new to delete, as expected.
What's left in the nix store is everything being referenced from the GC roots.
Let's install for a moment bsd-games:
$ nix-env -iA nixpkgs.bsdgames $ readlink -f `which fortune` /nix/store/b3lxx3d3ggxcggvjw5n0m1ya1gcrmbyn-bsd-games-2.17/bin/fortune $ nix-store -q --roots `which fortune` /nix/var/nix/profiles/default-9-link $ nix-env --list-generations [...] 9 2014-08-20 12:44:14 (current)
The nix-store command can be used to query the GC roots that refer to a given derivation. In this case, our current user environment does refer to bsd-games.
Now remove it, collect garbage and note that bsd-games is still in the nix store:
$ nix-env -e bsd-games uninstalling `bsd-games-2.17' $ nix-collect-garbage [...] $ ls /nix/store/b3lxx3d3ggxcggvjw5n0m1ya1gcrmbyn-bsd-games-2.17 bin share
This is because the old generation is still in the nix store because it's a GC root. As we'll see below, all profiles and their generations are GC roots.
Removing a GC root is simple. Let's try deleting the generation that refers to bsd-games, collect garbage, and note that now bsd-games is no longer in the nix store:
$ rm /nix/var/nix/profiles/default-9-link $ nix-env --list-generations [...] 8 2014-07-28 10:23:24 10 2014-08-20 12:47:16 (current) $ nix-collect-garbage [...] $ ls /nix/store/b3lxx3d3ggxcggvjw5n0m1ya1gcrmbyn-bsd-games-2.17 ls: cannot access /nix/store/b3lxx3d3ggxcggvjw5n0m1ya1gcrmbyn-bsd-games-2.17: No such file or directory
Note: nix-env --list-generations does not rely on any particular metadata. It is able to list generations based solely on the file names under the profiles directory.
However we removed the link from
/nix/var/nix/profiles, not from
/nix/var/nix/gcroots. Turns out, Nix
/nix/var/nix/profiles as a GC root.
That is very handy. It means any profile and its generations are GC roots.
There are other paths that are taken into account as well, for example
/run/booted-system on NixOS.
The command nix-store --gc --print-roots prints all the paths considered before performing a GC.
It's as simple as that, anything under
/nix/var/nix/gcroots is a GC root. And anything not
being garbage collected is because it's referred from one of the GC roots.
11.3. Indirect roots
Remember that building the GNU hello world package with
nix-build produces a
symlink in the current directory. Despite the collected garbage done
above, the hello program is still working: therefore
it has not been garbage collected. Clearly, since there's no other
derivation that depends upon the GNU hello world package, it must be a
In fact, nix-build automatically adds the result
symlink as a GC root. Yes, not the built derivation, but the symlink.
These GC roots are added under
$ ls -l /nix/var/nix/gcroots/auto/ total 8 drwxr-xr-x 2 nix nix 4096 Aug 20 10:24 ./ drwxr-xr-x 3 nix nix 4096 Jul 24 10:38 ../ lrwxrwxrwx 1 nix nix 16 Jul 31 10:51 xlgz5x2ppa0m72z5qfc78b8wlciwvgiz -> /home/nix/result/
Don't care about the name of the symlink. What's important is that a
symlink exists that point to
is called an indirect GC root. That is,
the GC root is effectively specified outside of
result points to, it will not be garbage collected.
How do we remove the derivation then? There are two possibilities:
Remove the indirect GC root from
In the first case, the derivation will be deleted from the nix store, and
result becomes a dangling symlink. In the second
case, the derivation is removed as well as the indirect root in
Running nix-collect-garbage after deleting the GC root or the indirect GC root, will remove the derivation from the store.
11.4. Cleanup everything
What's the main source of software duplication in the nix store? Clearly, GC roots due to nix-build and profile generations. Doing a nix-build results in a GC root for a build that somehow will refer to a specific version of glibc, and other libraries. After an upgrade, if that build is not deleted by the user, it will not be garbage collected. Thus the old dependencies referred to by the build will not be deleted either.
Same goes for profiles. Manipulating the nix-env profile will create further generations. Old generations refer to old software, thus increasing duplication in the nix store after an upgrade.
What are the basic steps for upgrading and removing everything old, including old generations? In other words, do an upgrade similar to other systems, where they forget everything about the older state:
$ nix-channel --update $ nix-env -u --always $ rm /nix/var/nix/gcroots/auto/* $ nix-collect-garbage -d
First, we download a new version of the nixpkgs channel, which holds the description of all the software. Then we upgrade our installed packages with nix-env -u. That will bring us into a fresh new generation with all updated software.
Then we remove all the indirect roots generated by nix-build: beware, this will result in dangling symlinks. You may be smarter and also remove the target of those symlinks.
Finally, the -d option of nix-collect-garbage is used to delete old generations of all profiles, then collect garbage. After this, you lose the ability to rollback to any previous generation. So make sure the new generation is working well before running the command.
Garbage collection in Nix is a powerful mechanism to cleanup your system. The nix-store commands allow us to know why a certain derivation is in the nix store.
Cleaning up everything down to the oldest bit of software after an upgrade seems a bit contrived, but that's the price of having multiple generations, multiple profiles, multiple versions of software, thus rollbacks etc.. The price of having many possibilities.
11.6. Next pill
...we will package another project and introduce what I call the "inputs" design pattern. We've only played with a single derivation until now, however we'd like to start organizing a small repository of software. The "inputs" pattern is widely used in nixpkgs; it allows us to decouple derivations from the repository itself and increase customization opportunities.