Journalling Flash File System (JFFS/JFFS2) is a re-writable area within a DD-WRT-enabled device and is located in /jffs. More on this in the dd-wrt official page
Enabling JFFS on Linksys WRT1900ACS v2 with DD-WRT
JFFS Setup
Navigate to Administration
Scroll down until you see JFFS2 Support section
Click Enable JFFS
Click Clean JFFS
Click Apply.
Note: The router formats the available space. After GUI comes up again establish an SSH connection to your router and check that /jffs is created
root@DD-WRT:~# cd /jffs root@DD-WRT:/jffs#