JFFS is a writeable section of the flash memory (the size will vary between router models, with the newer models having a bit over 60 MB of space 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). You can check the size and availability of the JFFS space in Tools > Sysinfo > Internal Storage.

Starting with 378.50, this option is enabled by default. If for some reason you need to erase its content, you can do so from the Administration page, under the System tab. Formatting the JFFS partition requires a reboot to take effect. Note that formatting it might possibly require a second reboot afterwards, if it fails to properly mount after that first reboot.

I do not recommend doing frequent writes to this area, as it may prematurely wear out the flash chip. 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 high activity 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.

Asus routers produced since the RT-AC68U use high endurance flash memory rated for 100,000 Program/Erase cycles so wear should not be an issue for most applications. Current Asus firmware now uses the JFFS partition to store not only logfiles but databases and other frequently updated information.

Backing up the JFFS Partition

Backing up the JFFS Partition is an easy thing to do in the firmware. It's recommended to back up if you're planning to update your firmware to a newer version or if you're changing scripts, configurations, or OpenVPN keys on your router. To back up JFFS, in the GUI go to:

Administration - Restore/Save/Upload Setting

Click the Save button next to Backup JFFS partition to back up the complete contents of the partition. To restore from a previous backup, click the Browse... button to the right of Restore JFFS partition to select the file from which to restore; then click the Upload button.

If you have a configuration or a script that you like, it's recommended that you store a JFFS partition backup file in case the router is reset or the partition is erased accidentally.

See also: NVRAM Save Restore Utility for backing up/restoring NVRAM Settings.