image: 2Mb default image size alignment

Change the default image size alignment from 128Mb to 2Mb. The guest
image size is 48Mb smaller after this change.

A custom size aligment can be specified using the new variable
IMAGE_SIZE_ALIGNMENT.

Using a smaller guest image size is currently significant on AKS-Kata
hosts, because the entire image gets copied into guest MAP_PRIVATE
pages when using cloud-hypervisor's "--pmem discard_writes=on".

Signed-off-by: Dan Mihai <[email protected]>

tools/osbuilder/image-builder/image_builder.sh

@@ -12,6 +12,7 @@ set -o pipefail
 
 DOCKER_RUNTIME=${DOCKER_RUNTIME:-runc}
 MEASURED_ROOTFS=${MEASURED_ROOTFS:-no}
+IMAGE_SIZE_ALIGNMENT_MB=${IMAGE_SIZE_ALIGNMENT_MB:-2}
 
 #For cross build
 CROSS_BUILD=${CROSS_BUILD:-false}
@@ -75,9 +76,6 @@ AGENT_INIT=${AGENT_INIT:-no}
 SELINUX=${SELINUX:-no}
 SELINUXFS="/sys/fs/selinux"
 
-# Align image to 128M
-readonly mem_boundary_mb=128
-
 # shellcheck source=../scripts/lib.sh
 source "${lib_file}"
 
@@ -327,9 +325,9 @@ calculate_img_size() {
 		img_size="$((img_size + root_free_space_mb))"
 	fi
 
-	remaining="$((img_size % mem_boundary_mb))"
+	remaining="$((img_size % ${IMAGE_SIZE_ALIGNMENT_MB}))"
 	if [ "${remaining}" != "0" ]; then
-		img_size=$((img_size + mem_boundary_mb - remaining))
+		img_size=$((img_size + ${IMAGE_SIZE_ALIGNMENT_MB} - remaining))
 	fi
 
 	echo "${img_size}"