Jffs

JFFS is a writable section of the flash memory (the size will vary between router models, it will usually be around 8 MB, except for the RT-AC66U which has a whole 32 MB available) which will allow you to store small files (such as scripts) inside the router without needing to have a USB disk plugged in. This space will survive reboot (but it might NOT survive firmware flashing, so back it up first before flashing!). It will also be available fairly early at boot (before USB disks).

To enable this option, go to the Administration page, under the System tab. First time you enable JFFS, it must be formatted. This can also be done on the same page. Enabling/Disabling/Formatting JFFS requires a reboot to take effect. Note that formatting it might require a second reboot afterwards.

I do not recommend doing frequent writes to this area, as it will prematurely wear out the flash RAM. This is a good place to put files that are written once like scripts or kernel modules, or that rarely get written to. Do not put files that get constantly written to (such as logfiles) - store these on a USB disk instead. Replacing a worn out USB flash disk is much cheaper than replacing the whole router if flash sectors get worn out - they have a limited number of write cycles.