Zero free space on rootfs so it's more compressible for auto-update

build_image

 #!/bin/bash
 
 # Copyright (c) 2009 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Script to build a bootable keyfob-based chromeos system image from within
 # a chromiumos setup. This assumes that all needed packages have been built into
 # the given target's root with binary packages turned on. This script will
 # build the Chrome OS image using only pre-built binary packages.
@@ skipping 485 matching lines @@
   # Clean out unused packages
   sudo INSTALL_MASK="${INSTALL_MASK}" ${EMERGE_BOARD_CMD} \
     --root="${ROOT_FS_DIR}" --root-deps=rdeps \
     --usepkgonly --depclean ${EMERGE_JOBS}
 
   trap - EXIT
   ${SCRIPTS_DIR}/mount_gpt_image.sh -u -r "${ROOT_FS_DIR}" \
     -s "${STATEFUL_FS_DIR}" -e "${ESP_FS_DIR}"
 }
 
+zero_free_space() {
+  local fs_mount_point=$1
+  info "Zeroing freespace in ${fs_mount_point}"
+  sudo dd if=/dev/zero of="${fs_mount_point}/filler" oflag=sync bs=4096 || true
+  sudo rm -f "${fs_mount_point}/filler"
+  sudo sync
+}
+
 create_base_image() {
   local image_name=$1
 
   trap "cleanup && delete_prompt" EXIT
 
   # Create and format the root file system.
 
   # Check for loop device before creating image.
   LOOP_DEV=$(sudo losetup -f)
   if [ -z "${LOOP_DEV}" ] ; then
@@ skipping 160 matching lines @@
   setup_symlinks_on_root "/usr/local" "/var" "${STATEFUL_FS_DIR}"
 
   # cros_make_image_bootable will clobber vmlinuz.image for x86.
   # Until then, just copy the kernel to vmlinuz.image. It is
   # expected in build_gpt.sh.
   cp "${ROOT_FS_DIR}/boot/vmlinuz" "${OUTPUT_DIR}/vmlinuz.image"
 
   # Create an empty esp image to be updated in by update_bootloaders.sh.
   ${SCRIPTS_DIR}/create_esp.sh --to="${ESP_FS_IMG}"
 
+  # Zero rootfs free space to make it more compressible so auto-update
+  # payloads become smaller
+  zero_free_space "${ROOT_FS_DIR}"
+
   cleanup
 
   trap delete_prompt EXIT
 
   # Create the GPT-formatted image.
   ${SCRIPTS_DIR}/build_gpt.sh \
     --arch=${ARCH} \
     --board=${FLAGS_board} \
     --rootfs_partition_size=${FLAGS_rootfs_partition_size} \
     "${OUTPUT_DIR}" \
@@ skipping 106 matching lines @@
   echo "Developer image created as ${DEVELOPER_IMAGE_NAME}"
 fi
 
 print_time_elapsed
 
 echo "To copy to USB keyfob, do something like:"
 echo "  ./image_to_usb.sh --from=${OUTSIDE_OUTPUT_DIR} --to=/dev/sdX"
 echo "To convert to VMWare image, INSIDE the chroot, do something like:"
 echo "  ./image_to_vm.sh --from=${OUTSIDE_OUTPUT_DIR} --board=${BOARD}"
 echo "from the scripts directory where you entered the chroot."