work with sparse images where possible

This speeds up the conversion quite a lot, meaning that it will skip
copying large blocks of 'zero' data back and forth.

Converting a 2018-11-13-raspbian-stretch-lite.img went from 8m10s to
4m45s on my machine.

Changelog: None

Signed-off-by: Mirza Krak <mirza.krak@northern.tech>

convert-stage-3.sh

@@ -7,7 +7,7 @@ build_log=$output_dir/build.log
 [ ! -f ${output_dir}/rootfs.img ] && \
     { log "Error: extracted rootfs partition not found. Aborting."; exit 1; }
 
-sudo dd if=${output_dir}/rootfs.img of=/dev/mapper/${rootfs_mapping} bs=8M >> "$build_log" 2>&1
+sudo dd if=${output_dir}/rootfs.img of=/dev/mapper/${rootfs_mapping} bs=8M conv=sparse >> "$build_log" 2>&1
 sync
 # Check Linux ext4 file system just in case.
 sudo fsck.ext4 -fp /dev/mapper/${rootfs_mapping} >> "$build_log" 2>&1

mender-convert

@@ -297,7 +297,7 @@ do_raw_disk_image_create_partitions() {
   calculate_mender_disk_size $sector_size ${partition_alignment} \
                              mender_disk_sizes mender_disk_image_size
 
-  log "\tCreating Mender disk image:\
+  log "\tCreating blank Mender disk image:\
        \n\t\timage size: ${mender_disk_image_size} bytes\
        \n\t\tboot partition size: $((${mender_disk_sizes[pboot_size]} * $sector_size)) bytes\
        \n\t\troot filesystem size: $((${mender_disk_sizes[prootfs_size]} * $sector_size)) bytes\

mender-convert-functions.sh

@@ -396,10 +396,9 @@ unmount_partitions() {
 create_mender_disk() {
   local lfile=$1
   local lsize=$2
-  local bs=$(( 1024*1024 ))
-  local count=$(( ${lsize} / ${bs} ))
 
-  dd if=/dev/zero of=${lfile} bs=${bs} count=${count}>> "$build_log" 2>&1
+  # Generates a sparse image
+  dd if=/dev/zero of=${lfile} seek=${lsize} bs=1 count=0 >> "$build_log" 2>&1
 }
 
 # Takes following arguments:
@@ -738,7 +737,7 @@ set_fstab() {
 
 extract_file_from_image() {
   log "\tStoring data in $4."
-  local cmd="dd if=$1 of=${output_dir}/$4 skip=$2 bs=512 count=$3"
+  local cmd="dd if=$1 of=${output_dir}/$4 skip=$2 bs=512 count=$3 conv=sparse"
   $(${cmd}>> "$build_log" 2>&1)
 }
 

qemux86_64-convert-stage-2.sh

@@ -14,7 +14,7 @@ sed -i 's/\b[ ]root=[^ ]*/ root=\/dev\/hda2/' ${output_dir}/grub.cfg
 # Update Linux kernel command arguments with our custom configuration
 mcopy -o -i ${output_dir}/boot.vfat -s ${output_dir}/grub.cfg ::EFI/BOOT/grub.cfg
 
-sudo dd if=${output_dir}/boot.vfat of=/dev/mapper/${boot_mapping} bs=1M >> "$build_log" 2>&1
+sudo dd if=${output_dir}/boot.vfat of=/dev/mapper/${boot_mapping} bs=1M conv=sparse >> "$build_log" 2>&1
 
 log "\tDone."
 

rpi-convert-stage-2.sh

@@ -18,7 +18,7 @@ mcopy -on -i ${output_dir}/boot.vfat -s ::config.txt ${output_dir}/config.txt
 echo -e '\nenable_uart=1\n' >> ${output_dir}/config.txt
 mcopy -o -i ${output_dir}/boot.vfat -s ${output_dir}/config.txt ::config.txt
 
-sudo dd if=${output_dir}/boot.vfat of=/dev/mapper/${boot_mapping} bs=1M >> "$build_log" 2>&1
+sudo dd if=${output_dir}/boot.vfat of=/dev/mapper/${boot_mapping} bs=1M conv=sparse >> "$build_log" 2>&1
 
 log "\tDone."