This is to try and squeeze more lost space from the image, so that hydra
starts building it again.
The fsck previous to the resize2fs is required so resize2fs works.
The one afterwards is a sanity check.
Using `-M` from resize2fs will not give much saved space due to a known
(in the manual) issue.
```
[samueldr@aarch64:~/nixpkgs]$ ls -lh result-*/*/*.img
-r--r--r-- 1 root root 2.2G Jan 1 1970 result-original/sd-image/nixos-sd-image-18.09.git.a7fd431-aarch64-linux.img
-r--r--r-- 1 root root 2.1G Jan 1 1970 result-M/sd-image/nixos-sd-image-18.09.git.a7fd431-aarch64-linux.img
-r--r--r-- 1 root root 1.9G Jan 1 1970 result-slimmed/sd-image/nixos-sd-image-18.09.git.a7fd431-aarch64-linux.img
```
```
[samueldr@aarch64:~/nixpkgs]$ nix path-info -S ./result-original
/nix/store/c8k9n78gylx293rjh762fr05a069kxp2-nixos-sd-image-18.09.git.a7fd431-aarch64-linux.img 3844125000
[samueldr@aarch64:~/nixpkgs]$ nix path-info -S ./result-slimmed
/nix/store/962238skj5mnzhrsmjy23dyzmxk77sp4-nixos-sd-image-18.09.git.a7fd431-aarch64-linux.img 3447473208
```
I ended up with a corrupted image with the debugfs contraption once, and
given I couldn't reproduce the problem I suppose that happens if the
filesystem of the builder runs out of space.
At least in this instance fsck could detect it, so let's add it as a
sanity check.
exportReferencesGraph is deprecated and doesn't have the generated
initial Nix database contain the SHA256 of the contents of the store
paths, which breaks various things under Nix 2.0.
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>'
