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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
87 lines
3 KiB
Nix
87 lines
3 KiB
Nix
# Builds an ext4 image containing a populated /nix/store with the closure
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# of store paths passed in the storePaths parameter, in addition to the
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# contents of a directory that can be populated with commands. The
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# generated image is sized to only fit its contents, with the expectation
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# that a script resizes the filesystem at boot time.
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{ pkgs
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, lib
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# List of derivations to be included
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, storePaths
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# Whether or not to compress the resulting image with zstd
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, compressImage ? false, zstd
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# Shell commands to populate the ./files directory.
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# All files in that directory are copied to the root of the FS.
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, populateImageCommands ? ""
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, volumeLabel
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, uuid ? "44444444-4444-4444-8888-888888888888"
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, e2fsprogs
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, libfaketime
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, perl
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, fakeroot
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}:
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let
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sdClosureInfo = pkgs.buildPackages.closureInfo { rootPaths = storePaths; };
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in
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pkgs.stdenv.mkDerivation {
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name = "ext4-fs.img${lib.optionalString compressImage ".zst"}";
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nativeBuildInputs = [ e2fsprogs.bin libfaketime perl fakeroot ]
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++ lib.optional compressImage zstd;
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buildCommand =
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''
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${if compressImage then "img=temp.img" else "img=$out"}
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(
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mkdir -p ./files
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${populateImageCommands}
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)
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echo "Preparing store paths for image..."
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# Create nix/store before copying path
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mkdir -p ./rootImage/nix/store
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xargs -I % cp -a --reflink=auto % -t ./rootImage/nix/store/ < ${sdClosureInfo}/store-paths
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(
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GLOBIGNORE=".:.."
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shopt -u dotglob
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for f in ./files/*; do
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cp -a --reflink=auto -t ./rootImage/ "$f"
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done
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)
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# Also include a manifest of the closures in a format suitable for nix-store --load-db
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cp ${sdClosureInfo}/registration ./rootImage/nix-path-registration
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# Make a crude approximation of the size of the target image.
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# If the script starts failing, increase the fudge factors here.
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numInodes=$(find ./rootImage | wc -l)
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numDataBlocks=$(du -s -c -B 4096 --apparent-size ./rootImage | tail -1 | awk '{ print int($1 * 1.10) }')
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bytes=$((2 * 4096 * $numInodes + 4096 * $numDataBlocks))
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echo "Creating an EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks)"
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truncate -s $bytes $img
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faketime -f "1970-01-01 00:00:01" fakeroot mkfs.ext4 -L ${volumeLabel} -U ${uuid} -d ./rootImage $img
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export EXT2FS_NO_MTAB_OK=yes
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# I have ended up with corrupted images sometimes, I suspect that happens when the build machine's disk gets full during the build.
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if ! fsck.ext4 -n -f $img; then
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echo "--- Fsck failed for EXT4 image of $bytes bytes (numInodes=$numInodes, numDataBlocks=$numDataBlocks) ---"
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cat errorlog
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return 1
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fi
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# We may want to shrink the file system and resize the image to
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# get rid of the unnecessary slack here--but see
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# https://github.com/NixOS/nixpkgs/issues/125121 for caveats.
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if [ ${builtins.toString compressImage} ]; then
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echo "Compressing image"
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zstd -v --no-progress ./$img -o $out
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fi
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'';
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}
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