The root filesystem resizing step, `resize2fs -M', does not provide any
control over the amount of slack left in the result. It can produce an
arbitrarily tight fit, depending on how well the payload aligns with
ext4 data structures.
This is problematic, as NixOS must create a few files and directories
during its first boot, before the root is enlarged to match the size of
the containing SD card.
An overly tight fit can cause failures in the first stage:
mkdir: can't create directory '/mnt-root/proc': No space left on device
or in the second stage:
install: cannot create directory '/var': No space left on device
A previous version of `make-ext4-fs' (before PR #79368) was explicitly
"reserving" 16 MiB of free space in the final filesystem. Manually
calculating the size of an ext4 filesystem is a perilous endeavor,
however, and the method it employed was apparently unreliable.
Reverting is consequently not a good option.
A solution would be to create some sort of "balloon" occupying inodes
and blocks in the image prior to invoking `resize2fs -M', and to remove
these temporary files/directories before the compression step.
This changeset takes the simpler approach of simply dropping the
resizing step.
Note that this does *not* result in a larger image in general, as the
current procedure does not truncate the `.img' file anyway. In fact, it
has been observed to yield *smaller* compressed images---probably
because of some "noise" left after resizing. E.g., before-vs-after:
-r--r--r-- 2 root root 607M 1. Jan 1970 nixos-sd-image-21.11pre-git-x86_64-linux.img.zst
-r--r--r-- 2 root root 606M 1. Jan 1970 nixos-sd-image-21.11pre-git-x86_64-linux.img.zst
In 9ac1ab10c9 this library function was
refactored to use mkfs.ext4 instead of cptofs. There are two problems:
If populateImageCommands would create no files (which is fine), a cp
invocation would fail due to missing source arguments.
Another problem is that mkfs.ext4 relies on fakeroot to have sane
uid/gids in the generated filesystem image. This currently doesn't
work for cross compiling.
Turns out, on smaller images (~800MiB uncompressed sdcard image size),
the current fudge factor is way too small to even get the system to the
phase where it can resize itself.
I first tried with 1.05, but it wasn't enough.
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>'
