OpenWrt File System Hierarchy / Memory Usage

OpenWrt File System Hierarchy
	Flash Storage Partitioning (TP-Link WR1043ND)					Main Memory Usage
Hardware	m25p80 spi0.0: m25p64 8192 KiB					main memory 32 768 KiB
Layer1	mtd0 *u-boot* 128 KiB	mtd5 *firmware* 8000 KiB			mtd4 *art* 64 KiB	Kernel space 3828 KiB	User space 28 940 KiB
Layer2		mtd1 *kernel* 1280 KiB	mtd2 *rootfs* 6720 KiB				up to 50%	512 KiB	remaining
mountpoint			/
filesystem			overlayfs
Layer3			1536 KiB	mtd3 *rootfs_data* 5184 KiB
mountpoint	none	none	/rom	/overlay	none		/tmp	/dev
filesystem	none	none	SquashFS	JFFS2	none		tmpfs	tmpfs

/ this is your entire root filesystem, it comprises /rom and /overlay. Please ignore /rom and /overlay and use exclusively / for your daily routines!
/rom contains all the basic files, like busybox, dropbear or iptables. It also includes default configuration files used when booting into OpenWrt Failsafe mode. It does not contain the Linux kernel. All files in this directory are located on the SqashFS partition, and thus cannot be altered or deleted. But, because we use overlay_fs filesystem, so called overlay-whiteout-symlinks can be created on the JFFS2 partition.
/overlay is the writable part of the file system that gets merged with /rom to create a uniform /-tree. It contains anything that was written to the router after installation, e.g. changed configuration files, additional packages installed with opkg, etc. It is formated with JFFS2.
Rather than deleting the files, insert a whiteout, a special high-priority entry that marks the file as deleted. File system code that sees a whiteout entry for file F behaves as if F does not exist.
```
#!/bin/sh
# shows all overlay-whiteout symlinks in the directory /overlay
 
find /overlay -type l | while read FILE
  do
    [ -z "$FILE" ] && break
    if ls -la "$FILE" 2>&- | grep -q '(overlay-whiteout)'; then
    echo "$FILE"
    fi
  done
```

/tmp is a tmpfs-partition

#!/bin/sh
# shows current size of the tmpfs-partition mounted to /tmp
calc_tmpfs_size() {pi_size=$(awk '/MemTotal:/ {l=10485760;mt=($2*1024);print((s=mt/2)<l)&&(mt>l)?mt-l:s}' /proc/meminfo)}}
echo $pi_size

/dev Driver Core: devtmpfs - kernel-maintained tmpfs-based /dev

History

early OpenWrt-versions: the rootfs was readonly and only NVRAM-variables could be edited
symlink approach
mini_fo r3667 add mini_fo patches to mount_root and firstboot and r3669 mini_fo works and r3928 enabled mini_fo by default
overlayfs r26209 kernel: replace mini_fo with overlayfs for 2.6.37 and r26213 kernel: replace mini_fo with overlayfs for 2.6.38

OpenWrt File System Hierarchy / Memory Usage

Mount Points

History