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_freespace() {

[petkov, 2010/12/11 00:25:44]

zero_free_space?

[thieule, 2010/12/11 00:54:35]

Done.

+  local fs_mount_point=$1
+  info "Zeroing freespace in ${fs_mount_point}"
+  sudo dd if=/dev/zero of="${fs_mount_point}/filler" oflag=sync || \

[petkov, 2010/12/11 00:25:44]

this seems strange, especially with || instead of &&
it seems to me you do want to execute dd || true
then sudo rm -f filler
then sudo sync

do you need oflag=sync?

have you tried non-default bs= values to improve performance?

[thieule, 2010/12/11 00:54:35]

Done.

The oflag is required to force the writes to disk instead of residing in cache. 
Since we delete it immediately, if we don't synchronously write to disk during
dd, the zeroes may not make it all the way to disk.

As for the bs= value, you've brought up an interesting point.  I've changed it
to explicitly use bs=4096 to match the file system block size.  This value is a
good trade off between performance and completeness.  Smaller values will lower
performance while higher values may result in some blocks not getting zero'ed.

+    sudo rm "${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
+  # packages become smaller

[petkov, 2010/12/11 00:25:44]

s/packages/payloads/

[thieule, 2010/12/11 00:54:35]

Done.

+  zero_freespace "${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."