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