cros_make_image_bootable, mount_gpt_image, common.sh: root filesystem changes: ext2, ro by default

build_image

Index: build_image
diff --git a/build_image b/build_image
index 3b3f061df6604739040167e80dacd7a0d5bc461a..9d5749e69ed8e3ff22715e2953facd2111100000 100755
--- a/build_image
+++ b/build_image
@@ -476,8 +476,6 @@ create_base_image() {
 
   trap "cleanup && delete_prompt" EXIT
 
-  UUID=$(uuidgen)
-
   # Create and format the root file system.
 
   # Check for loop device before creating image.
@@ -498,13 +496,22 @@ create_base_image() {
      seek=$((ROOT_SIZE_BYTES + ROOT_HASH_PAD - 1))
   sudo losetup "${LOOP_DEV}" "${ROOT_FS_IMG}"
   # Specify a block size and block count to avoid using the hash pad.
-  sudo mkfs.ext3 -b 4096 "${LOOP_DEV}" "$((ROOT_SIZE_BYTES / 4096))"
+  sudo mkfs.ext2 -b 4096 "${LOOP_DEV}" "$((ROOT_SIZE_BYTES / 4096))"
 
   # Tune and mount rootfs.
-  # TODO(wad) rename the disk label to match the GPT since we
-  #           can't change it later.
-  DISK_LABEL="C-KEYFOB"
-  sudo tune2fs -L "${DISK_LABEL}" -U "${UUID}" -c 0 -i 0 "${LOOP_DEV}"
+  DISK_LABEL="C-ROOT"
+  # Disable checking and minimize metadata differences across builds
+  # and wasted reserved space.
+  sudo tune2fs -L "${DISK_LABEL}" \
+               -U clear \
+               -T 20091119110000 \
+               -c 0 \
+               -i 0 \
+               -m 0 \

[gauravsh, 2010/10/20 19:06:55]

do you expect any performance effected from changing the reserved block
percentage from the default of 5% to 0%?

[Will Drewry, 2010/10/20 19:11:49]

I actually just wanted to make sure we use our filesystem to the fullest and not
keep extra blocks laying around for no reason on a ro filesystem.  That said, I
don't think it will have much impact except when we are very close to filling
the fs.  Thieu Le is also working on making the filesystem layout more
consistent across builds -- that will be more beneficial :)

From the man page:
"""
       -m reserved-blocks-percentage
              Set the percentage of the filesystem which may  only  be 
allocated  by  privileged  processes.
              Reserving  some  number  of  filesystem blocks for use by
privileged processes is done to avoid
              filesystem fragmentation, and to allow system daemons, such as
syslogd(8), to continue to func-
              tion  correctly  after  non-privileged  processes are prevented
from writing to the filesystem.
              Normally, the default percentage of reserved blocks is 5%.

"""

+               -r 0 \
+               -e remount-ro \
+                "${LOOP_DEV}"
+  # TODO(wad) call tune2fs prior to finalization to set the mount count to 0.
   sudo mount "${LOOP_DEV}" "${ROOT_FS_DIR}"
 
   # Create stateful partition of the same size as the rootfs.