Start with an appropriate image, such as core-image-minimal, analyze the dependencies as shown in the Debugging the build system recipe in Chapter 1, The Build System, and decide which of them are not needed. You could also use the file sizes listed in the image's build history, as seen in the Using build history recipe, also in Chapter 1, The Build System, to detect the biggest files in the filesystem and review them. To sort the file sizes, which appear in the fourth column of the files-in-image.txt file, in reverse order, we could execute:
$ sort -r -g -k 4,4 files-in-image.txt -o sorted-files-in-image.txt sorted-files-in-image.txt: -rw-r--r-- root root 7267688 ./boot/zImage-4.1.44+g0d698de42426 -rwxr-xr-x root root 2667248 ./usr/bin/trace -rwxr-xr-x root root 2667248 ./usr/bin/perf -r-xr-xr-x root root 1627632 ./usr/lib/libperl.so.5.24.1 -rwxr-xr-x root root 1261024 ./usr/lib/libstdc++.so.6.0.24 -rwxr-xr-x root root 1222352 ./lib/libc-2.26.so -rwxr-xr-x root root 867168 ./bin/bash.bash -rw-r--r-- root root 830424 ./usr/lib/libslang.so.2.3.1 -rwxr-xr-x root root 797648 ./usr/lib/libbfd-2.29.0.20170912.so -rwxr-xr-x root root 535396 ./bin/busybox.nosuid -rwxr-xr-x root root 525488 ./sbin/ldconfig -rwxr-xr-x root root 460144 ./lib/libm-2.26.so -rw-r--r-- root root 338696 ./boot/u-boot-wandboard-v2017.09+gitAUTOINC+a6a15fedd1-r0.img -rwxr-xr-x root root 265796 ./usr/bin/trace-cmd -rwxr-xr-x root root 263592 ./usr/lib/libpcre.so.1.2.9 -rwxr-xr-x root root 256356 ./usr/bin/udevadm -rwxr-xr-x root root 248212 ./sbin/udevd
Poky has a dirsize.py script that reports component sizes for the filesystem. An extract from its output (when launched from the rootfs directory in the tmp directory) is:
$ cd /opt/yocto/fsl-community-bsp/wandboard/tmp/work/wandboard-poky-linux-gnueabi/core-image-minimal/1.0-r0/rootfs
$ /opt/yocto/fsl-community-bsp/sources/poky/scripts/tiny/dirsize.py
12997690 .
7943835 ./boot
197 ./boot/extlinux
197 ./boot/extlinux/extlinux.conf
7267688 ./boot/zImage-4.1.44+g0d698de42426
338696 ./boot/u-boot-wandboard-v2017.09+gitAUTOINC+a6a15fedd1-r0.img
48128 ./boot/SPL-wandboard-v2017.09+gitAUTOINC+a6a15fedd1-r0
43827 ./boot/imx6qp-wandboard-revd1.dtb
42197 ./boot/imx6q-wandboard-revd1.dtb
41167 ./boot/imx6q-wandboard.dtb
41167 ./boot/imx6q-wandboard-revb1.dtb
40956 ./boot/imx6dl-wandboard-revd1.dtb
39906 ./boot/imx6dl-wandboard.dtb
39906 ./boot/imx6dl-wandboard-revb1.dtb
2958219 ./lib
From this, we observe that the boot folder, which contains u-boot, the kernel, the device tree, and the extlinux.conf used to boot the system, is the biggest contributor to filesystem size.
Some other places where some space on a console-only system can be saved are:
- Use the IPK package manager, as it is the lightest, or better yet remove the package-management feature from your production root filesystem altogether
- Use BusyBox's mdev device manager instead of udev by specifying it in your conf/local.conf file as follows:
VIRTUAL-RUNTIME_dev_manager = "busybox-mdev"
Note that this will only work with core images that include packagegroup-core-boot
- If we are running the root filesystem on a block device, use ext2 instead of ext3 or ext4 without the journal
- Configure BusyBox with only the essential applets by providing your own configuration file in a bbappend file
- Review the glibc configuration, which can be changed via the DISTRO_FEATURES_LIBC distribution configuration variable
- Compile your applications with -Os to optimize for size
- If using the Poky distribution, consider switching to a lighter C library than the default glibc such as musl, a new MIT-licensed C library
To enable the musl C library, you would add the following to your image file:
TCLIBC = "musl".