This option requests compatibility with older NixOS releases with
respect to stateful data, in cases where new releases have defaults
that might be incompatible with system state of existing NixOS
deployments. For instance, if we change the default version of
PostgreSQL, existing deployments will break if the new version can't
read databases created by the old version.
So for example, setting
system.stateVersion = "15.07";
requests that options like services.postgresql.package use defaults
corresponding to the 15.07 release branch. Note that
nixos-generate-config emits this option. (In the future, NixOps may
set system.stateVersion to the NixOS release in use when the machine
was created.)
See also #7939 for another motivating example.
The resulting image can be copied to a SD card with `dd` and is directly
bootable by a suitably configured U-Boot. Though depending on the board, some
extra steps are required for copying U-Boot itself to the SD card.
Inside the image is a partition table, with a FAT32 /boot and a normal
writable EXT4 rootfs. It's possible to directly reuse the SD image's
partition layout and "install" NixOS on the same SD card by replacing
the default configuration.nix and nixos-rebuild, and actually is the
preferred way to use these images. To assist in this installation
method, the boot scripts on the image automatically resize the rootfs
partition to fit the SD card on the first boot.
The SD images come in two flavors; one for the ARMv6 Raspberry Pi,
and one multiplatform image for all the boards supported by the
mainline kernel's multi_v7_defconfig config target. At the moment, these
have been tested on:
- Raspberry Pi Model B (512MB model)
- NVIDIA Jetson TK1
- Linksprite pcDuino3 Nano
To build, run:
nix-build '<nixpkgs/nixos>' -A config.system.build.sdImage \
-I nixos-config='<nixpkgs/nixos/modules/installer/cd-dvd/sd-image-armv7l-multiplatform.nix>'
When using extlinux-conf-builder in a nix build using chroots, the
following error message could be seen:
/nix/store/XXX-extlinux-conf-builder.sh: line 121: cd: /nix/var/nix/profiles: No such file or directory
To avoid this, just skip the code path parsing /nix/var/nix/profiles
when $numGenerations (passed from the command line) is 0 (which is the
only legal value of $numGenerations in a nix build context).
The rationale for disabling this is: 1) systemd timers are better; 2)
it gets rid of one usually unnecessary process, which makes containers
more light-weight.
Note that cron is still enabled if services.cron.systemCronJobs is
non-empty, so this only matters if you have no declarative cron jobs
but do have user cron jobs.
Without a menu title, U-Boot's distro scripts just autoboot the first
entry by default.
When I initially wrote this, my board wasn't apparently running stock
U-Boot but had some local hacks saved in the U-Boot's environment
which made it always display the prompt.
When calling addEntry inside a subshell, the filesCopied array would
be updated only in the subshell's environment. This would only cause an
issue if no -g flag was passed to the script, causing no kernels
to be copied.
