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

build_image

Index: build_image
diff --git a/build_image b/build_image
index 722d704f96f5ad70ccc30b8af16b78afa1dad360..1616d5dd349708506646b50f92473b85c1dee2da 100755
--- a/build_image
+++ b/build_image
@@ -503,6 +503,13 @@ update_base_packages() {
     -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
 
@@ -683,6 +690,10 @@ create_base_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