sd-image: Pull less slack in the image by accounting for slack

The slack, seemingly, accounted for more than the minimum required for
slack plus the two partitions.

This change makes the gap a somewhat abstracted amount, but is not
configurable within the derivation.

1 files changed, 7 insertions, 4 deletions

diff --git a/nixos/modules/installer/cd-dvd/sd-image.nix b/nixos/modules/installer/cd-dvd/sd-image.nix
index 1faa657885b..5ee594b2336 100644
--- a/nixos/modules/installer/cd-dvd/sd-image.nix
+++ b/nixos/modules/installer/cd-dvd/sd-image.nix
@@ -125,10 +125,13 @@ in
         echo "${pkgs.stdenv.buildPlatform.system}" > $out/nix-support/system
         echo "file sd-image $img" >> $out/nix-support/hydra-build-products
 
-        # Create the image file sized to fit /boot/firmware and /, plus 20M of slack
+        # Gap in front of the first partition, in MiB
+        gap=8
+
+        # Create the image file sized to fit /boot/firmware and /, plus slack for the gap.
         rootSizeBlocks=$(du -B 512 --apparent-size ${rootfsImage} | awk '{ print $1 }')
         firmwareSizeBlocks=$((${toString config.sdImage.firmwareSize} * 1024 * 1024 / 512))
-        imageSize=$((rootSizeBlocks * 512 + firmwareSizeBlocks * 512 + 20 * 1024 * 1024))
+        imageSize=$((rootSizeBlocks * 512 + firmwareSizeBlocks * 512 + gap * 1024 * 1024))
         truncate -s $imageSize $img
 
         # type=b is 'W95 FAT32', type=83 is 'Linux'.
@@ -138,8 +141,8 @@ in
             label: dos
             label-id: ${config.sdImage.firmwarePartitionID}
 
-            start=8M, size=$firmwareSizeBlocks, type=b
-            start=${toString (8 + config.sdImage.firmwareSize)}M, type=83, bootable
+            start=''${gap}M, size=$firmwareSizeBlocks, type=b
+            start=$((gap + ${toString config.sdImage.firmwareSize}))M, type=83, bootable
         EOF
 
         # Copy the rootfs into the SD image