Flash space is scarce on devices with only 4MB flash. You can save some space while compiling your own image (e.g. with the imagebuilder) by removing packages that are not needed for your usecase. In order to completely remove IPv6 support and related packages you also need to ask the imagebuilder to do so by also passing this option to the make command: CONFIG_IPV6=n.

Do not use zram-swap for 4MB flash devices as it increases the amount of firmware space used. It is listed here as it is helpful on machines with very little RAM memory.

The minus -PACKAGE_NAME prefix removes a package, no prefix adds one.

Example image builder command line

Note: In the command line shown below, you need to adjust PROFILE=tl-wr941nd-v6 to your device. Use make info to list possibilities.

Resources:

• https://forum.openwrt.org/t/can-i-strip-the-ipv6-out-with-the-imagebuilder/3465/5
• https://forum.openwrt.org/t/build-lede-without-ipv6/2043
• https://forum.openwrt.org/t/make-image-problem/898
• https://forum.openwrt.org/t/tp-link-tl-mr-3420-v3-build/855/8
• https://forum.openwrt.org/t/possible-ppp-removal-from-base-image/42024/3
• Talk Fight for the bytes! Fun with Four Megabytes Flash

If the device has 16MB or less of RAM, then it should only be used as Internal AP (e.g. to extend Wi-Fi coverage).

Example image builder command line

Note: In the command line shown below, you need to adjust PROFILE=tl-wr941nd-v6 to your device. Use make info to list possibilities.

Resources:

• https://forum.openwrt.org/t/zram-configuration/3560
• https://forum.openwrt.org/t/best-practices-for-32-mb-ram/5050
• https://forum.openwrt.org/t/luci-slow-down-to-death-wr1043n-v1/1992

This option compiles the kernel modules inside the kernel, you can either pick what modules you think can be integrated and make them built-in by pressing “y” on them when in the menuconfig, or do a quick-and-dirty change of the kernel config with (adjust the path in this example to point to your target's actual kernel config)

sed -i -e "s/=m/=y/g" build_dir/target-mipsel_24kc_musl/linux-ramips_mt7620/linux-4.14.63/.config

This will work only if your device's kernel partition is big enough to accomodate the slightly larger kernel, or if the device is using dynamic partitions.

This was reported by Daniel Santos in an email to the OpenWrt developer mailing list, 30 Oct 2018.

His comments:

I did a quick experiment of this and instead of saving 4k, my *image* is a full 256k smaller. I haven't analysed the specifics, but also this means less RAM consumed because squashfs uses the page cache for uncompressed files. Further, modules inherently have greater overhead, even after __init sections have been discarded. The only downside is that built-ins cannot be unloaded and will always occupy a portion of RAM. But having them built into the kernel is far more efficient.

(The “surprising” change in image size may be due to the block size of the squashfs image. See, for example include/image-commands.mk and include/image.mk in November, 2018 sources)

In a follow up mail by Phillip Prindeville pointed out the main drawback of this method.

Some hardware (it’s rare but not unheard of) can only be reset by unloading and reloading the module that controls it. Otherwise, you have to reboot the box. If you build all of your drivers in, then rebooting is all you have.

You can also save space by changing configuration variables using make menuconfig. In addition to the ones mentioned here you can save a tiny bit of space by disabling commands in busybox.

Config variable (n = disable, y = enable)	Menu path	Comments
CONFIG_KERNEL_PRINTK=n	Global build settings / Kernel build options / Enable support for printk
CONFIG_KERNEL_CRASHLOG=n	Global build settings / Kernel build options / Crash logging
CONFIG_KERNEL_SWAP=n	Global build settings / Kernel build options / Support for paging of anonymous memory (swap)
CONFIG_KERNEL_KALLSYMS=n	Global build settings / Kernel build options / Compile the kernel with symbol table information
CONFIG_KERNEL_DEBUG_INFO=n	Global build settings / Kernel build options / Compile the kernel with debug information
CONFIG_KERNEL_ELF_CORE=n	Global build settings / Kernel build options / Enable process core dump support
CONFIG_IPV6=n	Global build settings / Enable IPv6 support in packages
CONFIG_KERNEL_MAGIC_SYSRQ=n	Global build settings / Kernel build options / Compile the kernel with SysRq support
CONFIG_KERNEL_PRINTK_TIME=n	Global build settings / Kernel build options / Enable printk timestamps
CONFIG_PACKAGE_MAC80211_DEBUGFS=n	Kernel modules / Wireless Drivers / kmod-mac80211 / Export mac80211 internals in DebugFS	Estimated savings: 21 KB
CONFIG_PACKAGE_MAC80211_MESH=n	Kernel modules / Wireless Drivers / kmod-mac80211 / Enable 802.11s mesh support	Estimated savings: 29 KB
CONFIG_STRIP_KERNEL_EXPORTS=y	Global build settings / Strip unnecessary exports from the kernel image
CONFIG_USE_MKLIBS=y	Global build settings / Strip unnecessary functions from libraries
CONFIG_SERIAL_8250=n	Device Drivers / Character devices / Serial drivers / 8250/16550 and compatible serial support	This will also save RAM by preventing /sbin/askfirst from running. However, this will break sysupgrade. This menu is available only via make kernel_menuconfig
CONFIG_EARLY_PRINTK=n	Kernel hacking / Early printk	This menu is available only via make kernel_menuconfig
KERNEL_SQUASHFS_FRAGMENT_CACHE_SIZE	Global build settings / Kernel build options / Number of squashfs fragments cached	Reduce the number of cached blocks. This menu is available only via make kernel_menuconfig
TARGET_SQUASHFS_BLOCK_SIZE	Target Images / squashfs / Block size	Increase block size from the default 256 KB to improve compression

Block size must be a power of 2, between 4096 bytes and 1 Megabyte. By default, 3 blocks will be cached so this will also make the router use more RAM (if 3 blocks are cached, and a block size of 1024 KB is used, this will use 2,304 KB more RAM (3 * 1024 KB - 3 * 256 KB)). It may also make the router use more CPU to decompress the larger blocks.

To build a kernel that only works with a specific device, run make kernel_menuconfig CONFIG_TARGET=subtarget to edit target/linux/<platform>/*/config-default and go to Machine selection / <Platform> machine selection and disable all devices except for the specific device. This can save some disk space and RAM.

The icons in LuCI are not strictly necessary and replacing them with a blank image will save about 15 KB. This bash script will take an OpenWrt git path as parameter and replace the images with a blank using the files path:

#!/bin/sh
if [ -z "$1" ]; then echo Need git path as parameter; exit 1; fi
cd "$1" || exit 1
gitpath=`pwd`
echo Installing to $gitpath
mkdir -p files/www/luci-static/resources/icons/
mkdir -p files/www/luci-static/resources/cbi/
wget -O files/www/luci-static/resources/cbi/blank.gif 'https://raw.githubusercontent.com/mathiasbynens/small/master/gif-transparent.gif'
wget -O files/www/luci-static/resources/cbi/blank.png 'https://raw.githubusercontent.com/mathiasbynens/small/master/png-transparent.png'
cd feeds/luci/modules/luci-base/htdocs/luci-static/resources/cbi
for a in *.gif; do ln -s blank.gif "$gitpath"/files/www/luci-static/resources/cbi/$a; done
cd ../icons
for a in *.gif; do ln -s ../cbi/blank.gif "$gitpath"/files/www/luci-static/resources/icons/$a; done
for a in *.png; do ln -s ../cbi/blank.png "$gitpath"/files/www/luci-static/resources/icons/$a; done

• https://wiki.yoctoproject.org/wiki/Minimal_Image
• https://wiki.yoctoproject.org/wiki/Linux_kernel/Image_Size
• https://elinux.org/System_Size
• https://elinux.org/Runtime_Memory_Measurement
• https://tiny.wiki.kernel.org/
• https://github.com/yurt-page/docs