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## Background
Every now and then NixOS hosts runs out of disk space. This happened to my IRC
server recently...
> No problem. Let's free-up some space with Nix's garbage-collection:
> - me
```shell
λ nix-collect-garbage -d # failed due lack of disk space
```
Ironically Nix needs to do an SQLite transaction before deleting stuff and
SQLite can't do that if there's no space. This is especially funny because the
SQLite is probably a `DELETE`.
## Solution
First let's verify that our disk is indeed at capacity:
```shell
λ df -h
Filesystem Size Used Avail Use% Mounted on
devtmpfs 399M 0 399M 0% /dev
tmpfs 3.9G 0 3.9G 0% /dev/shm
tmpfs 2.0G 3.7M 2.0G 1% /run
tmpfs 3.9G 408K 3.9G 1% /run/wrappers
/dev/disk/by-label/nixos 9.9G 9.9G 0G 100% /
tmpfs 4.0M 0 4.0M 0% /sys/fs/cgroup
tmpfs 797M 0 797M 0% /run/user/0
```
Looks like `/dev/disk/by-label/nixos` is at `100%`. Now let's find some easy
targets to free-up space so that we can run `nix-collect-garbage -d`...
```shell
λ du -hs /* 2>/dev/null
8.0K /bin
12M /boot
0 /dev
200K /etc
68K /home
16K /lost+found
9.0G /nix
0 /proc
1.2M /root
2.9M /run
4.0K /srv
0 /sys
44K /tmp
12K /usr
1.2G /var
```
Okay: `/var` looks like an easy candidate. Let's recurse into that directory:
```shell
λ du -hs /var/*
40K /var/cache
12K /var/db
4.0K /var/empty
4.0K /var/google-users.d
211M /var/lib
0 /var/lock
918M /var/log
0 /var/run
4.0K /var/spool
44K /var/tmp
λ du -hs /var/log/* # /var/log looks promising
60M /var/log/btmp
82M /var/log/btmp.1
776M /var/log/journal # ah-ha! journald. Let's clean-up some logs
8.0K /var/log/lastlog
1.1M /var/log/nginx
4.0K /var/log/private
12K /var/log/wtmp
```
To retain at most 1w's worth of logs:
```shell
λ journalctl --vacuum-time=1w
```
...or if you'd prefer to retain only 100M's worth of logs:
```shell
λ journalctl --vacuum-size=100M
```
Now Nix should be able to garbage-collect!
```shell
λ nix-collect-garbage -d
```
And lastly verify that it WAI'd:
```
λ df -h
Filesystem Size Used Avail Use% Mounted on
devtmpfs 399M 0 399M 0% /dev
tmpfs 3.9G 0 3.9G 0% /dev/shm
tmpfs 2.0G 3.7M 2.0G 1% /run
tmpfs 3.9G 408K 3.9G 1% /run/wrappers
/dev/disk/by-label/nixos 9.9G 5.1G 4.3G 55% /
tmpfs 4.0M 0 4.0M 0% /sys/fs/cgroup
tmpfs 797M 0 797M 0% /run/user/0
```
## Closing Thoughts
Why doesn't Nix just reserve enough space to be able to GC itself? Not sure...
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