Use sparse files when generating image (#1326)

Use sparse files instead of files written full of zeros. This speeds up
the image generation process significantly. It also makes sure that
virtual disk image formats are minimal in size.

Note: qemu-img automatically generates sparse files when detecting a
block full of zeros. But this is applied on the write side, after image
convertion: The disk image format itself still thinks the whole image
is allocated, leading to larger image than necessary. Also some output
format seem to regonize chunks of zero and create sparse files themself.

With this change, the raw source image file is a sparse file. This is
regocnized by qemu-img at read time (see block/file-posix.c), and leads
to "native" sparse files in the output format.

Some numbers
- qcow2 1.8G -> 862M (same on-disk size)
- vdi 15G -> 888M (same on-disk size)
- vhdx 30G -> 1.1G (918M -> 861M on-disk size)
- vmdk 1.8G -> 866M (about the same on-disk size)

Obviously this also affects the compressed size. But because there are
still lots of zeros, the difference in compressed size is not that big.

buildroot-external/scripts/hdd-image.sh

@@ -56,7 +56,8 @@ function get_boot_size() {
 function create_spl_image() {
     local boot_img="$(path_spl_img)"
 
-    dd if=/dev/zero of="${boot_img}" bs=512 count=16382
+    rm -f "${boot_img}"
+    truncate --size=8M "${boot_img}"
 }
 
 
@@ -65,7 +66,8 @@ function create_boot_image() {
     local boot_img="$(path_boot_img)"
 
     echo "mtools_skip_check=1" > ~/.mtoolsrc
-    dd if=/dev/zero of="${boot_img}" bs="$(get_boot_size)" count=1
+    rm -f "${boot_img}"
+    truncate --size="$(get_boot_size)" "${boot_img}"
     mkfs.vfat -n "hassos-boot" "${boot_img}"
     mcopy -i "${boot_img}" -sv "${boot_data}"/* ::
 }
@@ -74,7 +76,8 @@ function create_boot_image() {
 function create_overlay_image() {
     local overlay_img="$(path_overlay_img)"
 
-    dd if=/dev/zero of="${overlay_img}" bs=${OVERLAY_SIZE} count=1
+    rm -f "${overlay_img}"
+    truncate --size="${OVERLAY_SIZE}" "${overlay_img}"
     mkfs.ext4 -L "hassos-overlay" -E lazy_itable_init=0,lazy_journal_init=0 "${overlay_img}"
 }
 
@@ -85,7 +88,8 @@ function create_kernel_image() {
     local kernel="${BINARIES_DIR}/${KERNEL_FILE}"
 
     # Make image
-    dd if=/dev/zero of="${kernel_img}" bs=${KERNEL_SIZE} count=1
+    rm -f "${kernel_img}"
+    truncate --size="${KERNEL_SIZE}" "${kernel_img}"
     mkfs.ext4 -L "hassos-kernel" -E lazy_itable_init=0,lazy_journal_init=0 -O ^extent,^64bit "${kernel_img}"
 
     # Mount / init file structs
@@ -131,7 +135,8 @@ function _create_disk_gpt() {
 
     ##
     # Write new image & GPT
-    dd if=/dev/zero of="${hdd_img}" bs=1G count="${hdd_count}"
+    rm -f "${hdd_img}"
+    truncate --size="${hdd_count}G" "${hdd_img}"
     sgdisk -o "${hdd_img}"
 
     ##
@@ -174,11 +179,11 @@ function _create_disk_gpt() {
     ##
     # Write Images
     sgdisk -v
-    dd if="${boot_img}" of"=${hdd_img}" conv=notrunc bs=512 seek="${boot_offset}"
-    dd if="${kernel_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${kernel_offset}"
-    dd if="${rootfs_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${rootfs_offset}"
-    dd if="${overlay_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${overlay_offset}"
-    dd if="${data_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${data_offset}"
+    dd if="${boot_img}" of"=${hdd_img}" conv=notrunc,sparse bs=512 seek="${boot_offset}"
+    dd if="${kernel_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${kernel_offset}"
+    dd if="${rootfs_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${rootfs_offset}"
+    dd if="${overlay_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${overlay_offset}"
+    dd if="${data_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${data_offset}"
 
     # Set Hyprid partition
     if [ "${BOOT_SYS}" == "hyprid" ]; then
@@ -230,7 +235,8 @@ function _create_disk_mbr() {
     local data_offset=${data_start}
 
     # Write new image & MBR
-    dd if=/dev/zero of="${hdd_img}" bs=1G count="${hdd_count}"
+    rm -f "${hdd_img}"
+    truncate --size="${hdd_count}G" "${hdd_img}"
 
     # Update disk layout
     (
@@ -252,11 +258,11 @@ function _create_disk_mbr() {
     sfdisk "${hdd_img}" < "${disk_layout}"
 
     # Write Images
-    dd if="${boot_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${boot_offset}"
-    dd if="${kernel_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${kernel_offset}"
-    dd if="${rootfs_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${rootfs_offset}"
-    dd if="${overlay_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${overlay_offset}"
-    dd if="${data_img}" of="${hdd_img}" conv=notrunc bs=512 seek="${data_offset}"
+    dd if="${boot_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${boot_offset}"
+    dd if="${kernel_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${kernel_offset}"
+    dd if="${rootfs_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${rootfs_offset}"
+    dd if="${overlay_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${overlay_offset}"
+    dd if="${data_img}" of="${hdd_img}" conv=notrunc,sparse bs=512 seek="${data_offset}"
 
     # Write SPL
     if [ "${BOOT_SPL}" == "true" ]; then