http://bandwidthd.sourceforge.net/

opkg install bandwidthd uhttpd
/etc/init.d/uhttpd enable
/etc/init.d/uhttpd start
/etc/init.d/bandwidthd enable
/etc/init.d/bandwidthd start

With all dependencies it uses around 500 kB of storage space

Bandwidthd uses a lot of cpu power to process and analyse packages, on average TP-Link TL-WR841ND router with 400 Mhz CPU it maxes out CPU with 100% usage around 3 Mpbs speeds. So if you have faster connection it throttles it down to around 3 Mbps because CPU can't handle more packages.

Note: As of build r31249 this is not necessarily true. While bandwidthd still maxes out at 100% cpu and >4 load average even while idle, the router will happily exceed 70mbps download speeds even with QoS on, which by itself uses >80% cpu at those speeds. In other words, even while bandwidthd is using 100% cpu, the above router will still easily exceed most people's connection speeds without the CPU becoming a limitation.

Edit /etc/config/bandwidthd

By default, bandwidthd hosts its statistics at /bandwidthd. For example, if the OpenWRT router's IP address is 192.168.0.1, bandwidthd's stats would be available at http://192.168.0.1/bandwidthd

The default bandwidthd installation loses your previous statistics on each reboot. To keep statistics between reboots it is needed to modify "/etc/config/bandwidthd" and "/etc/init.d/bandwidthd"

The only thing to understand is that bandwidthd binary, when launched, looks for the "httpd" folder and "bandwidthd.conf" file in the same folder where the binary has been lauched. Default /etc/init.d/bandwidthd init script creates two symlinks:

/httpd >> /www/bandwidthd
/bandwidthd.conf >> /etc/bandwidthd.conf 

then move to / folder and lauches there the bandwidthd binary.

These modifications change the init script behaviour to create httpd folder and bandwidthd.conf file in external storage.

Suppose we have a usb stick mounted in /mnt/usb and will store the bandwidthd stats in /mnt/usb/bandwidthd. Just modify the start() function in /etc/init.d/bandwidthd

start() {
        local subnet conffile
 
        [ -d /mnt/usb/bandwidthd/htdocs ] || mkdir -p /mnt/usb/bandwidthd/htdocs && cp /www/legend.gif /www/logo.gif /mnt/usb/bandwidthd/htdocs
        [ -e /www/bandwidthd ] || ln -s /mnt/usb/bandwidthd/htdocs /www/bandwidthd
        rm -f /mnt/usb/bandwidthd/bandwidthd.conf
        touch /mnt/usb/bandwidthd/bandwidthd.conf
        [ -e /etc/bandwidthd.conf ] || ln -s /mnt/usb/bandwidthd/bandwidthd.conf /etc/bandwidthd.conf
 
 
        config_load bandwidthd
        for cfg in $cfgs; do
                config_get subnets $cfg subnets
                        for subnet in $subnets; do
                                conffile="${conffile}subnet ${subnet}\n"
                        done
                export_string dev $cfg
                export_number skip_intervals $cfg
                export_number graph_cutoff $cfg
                export_bool promiscuous $cfg
                export_bool output_cdf $cfg
                export_bool recover_cdf $cfg
                export_string filter $cfg
                export_bool graph $cfg
                export_number meta_refresh $cfg
                [ -n "$conffile" ] && echo -e "$conffile" >>/mnt/usb/bandwidthd/bandwidthd.conf
                unset conffile
        done
        cd /mnt/usb/bandwidthd
        bandwidthd
}

Next modify /etc/config/bandwidthd file to save statistics to disk. You need to change "option output_cdf true" and "option recover_cdf true"

config bandwidthd
	option dev		eth2
	option subnets		"10.20.30.0/24"
	option skip_intervals	0
	option graph_cutoff	1024
	option promiscuous	true
	option output_cdf	true
	option recover_cdf	true
	option filter		ip
	option graph		true
	option meta_refresh	150

After modifying both of the files, restart the service. Afterwards, you will get a file and directory structure on your external mount like this:

root@openwrt:~# ls -la /mnt/usb/bandwidthd/
drwxr-xr-x    3 root     root         1024 Aug 25 12:21 .
drwxr-xr-x    6 root     root         1024 Aug 25 00:39 ..
-rw-r--r--    1 root     root          158 Aug 25 00:40 bandwidthd.conf
drwxr-xr-x    2 root     root         2048 Aug 25 00:40 htdocs
-rw-r--r--    1 root     root        50034 Aug 25 12:21 log.1.0.cdf
-rw-r--r--    1 root     root        22204 Aug 25 12:21 log.2.0.cdf
-rw-r--r--    1 root     root         6698 Aug 25 12:10 log.3.0.cdf
-rw-r--r--    1 root     root          529 Aug 25 00:39 log.4.0.cdf