nixpkgs/nixos/modules/system/boot/grow-partition.nix

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# This module automatically grows the root partition.
# This allows an instance to be created with a bigger root filesystem
# than provided by the machine image.
{ config, lib, pkgs, ... }:
with lib;
{
imports = [
(mkRenamedOptionModule [ "virtualisation" "growPartition" ] [ "boot" "growPartition" ])
];
options = {
boot.growPartition = mkEnableOption "grow the root partition on boot";
};
config = mkIf config.boot.growPartition {
boot.initrd.extraUtilsCommands = ''
copy_bin_and_libs ${pkgs.gawk}/bin/gawk
copy_bin_and_libs ${pkgs.gnused}/bin/sed
copy_bin_and_libs ${pkgs.utillinux}/sbin/sfdisk
copy_bin_and_libs ${pkgs.utillinux}/sbin/lsblk
substitute "${pkgs.cloud-utils}/bin/.growpart-wrapped" "$out/bin/growpart" \
--replace "${pkgs.bash}/bin/sh" "/bin/sh" \
--replace "awk" "gawk" \
--replace "sed" "gnused"
ln -s sed $out/bin/gnused
'';
boot.initrd.postDeviceCommands = ''
rootDevice="${config.fileSystems."/".device}"
if waitDevice "$rootDevice"; then
rootDevice="$(readlink -f "$rootDevice")"
parentDevice="$rootDevice"
while [ "''${parentDevice%[0-9]}" != "''${parentDevice}" ]; do
parentDevice="''${parentDevice%[0-9]}";
done
partNum="''${rootDevice#''${parentDevice}}"
if [ "''${parentDevice%[0-9]p}" != "''${parentDevice}" ] && [ -b "''${parentDevice%p}" ]; then
parentDevice="''${parentDevice%p}"
fi
TMPDIR=/run sh $(type -P growpart) "$parentDevice" "$partNum"
udevadm settle
fi
'';
};
}