Rootfs is the filesystem that the root directory (also called ‘/’) is on. The N900 has a 256 MB OneNAND chip that is used for the rootfs, bootloader and kernel, together with a (slower) 32 GB eMMC flash that is partitioned as 768 MB swap space, 2 GB /home and 27 GB /home/user/MyDocs.

Every application installs a bit of itself to the rootfs, some more than others, and the rest of it they install on other parts of the device. Applications that have been designed to install as little of themselves as they can are referred to as "optified". As it is small, the rootfs tends to fill up quickly, so one should be aware of it. A good way to keep track of the free space is to download Conky (or the desktop command widget) and check it once in a while. If you run out of space, you cannot install anything else. Firmware updates require huge amounts of rootfs space (just for installation) and updating can become extremely problematic. There are ways to free up rootfs space