fix(build_library): Re-enable zeroing free-space but in a sane way.

As of Linux 3.2 loopback supports discard by punching holes in the
underlying file. This doesn't actually seem to impact things right now
since we are writing to fresh filesystems but might as well do this to
prevent wasted space from sneaking in later on.

build_library/base_image_util.sh

@@ -36,17 +36,6 @@ cleanup_mounts() {
   set -e
 }
 
-zero_free_space() {
-  local fs_mount_point=$1
-  info "Zeroing freespace in ${fs_mount_point}"
-  # dd is a silly thing and will produce a "No space left on device" message
-  # that cannot be turned off and is confusing to unsuspecting victims.
-  info "${fs_mount_point}/filler"
-  ( sudo dd if=/dev/zero of="${fs_mount_point}/filler" bs=4096 conv=fdatasync \
-      status=noxfer || true ) 2>&1 | grep -v "No space left on device"
-  sudo rm "${fs_mount_point}/filler"
-}
-
 create_base_image() {
   local image_name=$1
   local rootfs_verification_enabled=$2
@@ -281,10 +270,10 @@ create_base_image() {
   # create /usr/local or /var on host (already exist on target).
   setup_symlinks_on_root "/usr/local" "/var" "${stateful_fs_dir}"
 
-  # Zero rootfs free space to make it more compressible so auto-update
-  # payloads become smaller
-  # TODO(ifup):
-  #zero_free_space "${root_fs_dir}"
+  # Zero all fs free space to make it more compressible so auto-update
+  # payloads become smaller, not fatal since it won't work on linux < 3.2
+  sudo fstrim "${root_fs_dir}" || true
+  sudo fstrim "${stateful_fs_dir}" || true
 
   cleanup_mounts
 

build_library/dev_image_util.sh

@@ -88,6 +88,10 @@ EOF
     sudo chmod a+rx "${path}"
   fi
 
+  # Zero all fs free space, not fatal since it won't work on linux < 3.2
+  sudo fstrim "${root_fs_dir}" || true
+  sudo fstrim "${stateful_fs_dir}" || true
+
   info "Developer image built and stored at ${image_name}"
 
   cleanup_mounts