romfs is a space-efficient, small, READ-ONLY filesystem originally for Linux and used by some Linux based projects. It is a block-based filesystem, that means it makes use of block (or sector) accessible storage driver (like disks, CDs, ROM drives, RAM disk, etc).
romfs itself, although it may sound weird at first, cannot handle ROMs of any kind. It is just a file system, working over a block device, including ROM disk or RAM disk. In order for romfs to work on ROM or RAM, you need drivers to make them appear as disk (ROM disk or RAM disk).
Where romfs is used:
There are at least two main uses where romfs, despite its limitations, is almost perfect.
1. The first is normal Linux systems, where you want some of the files to take the least possible space, and you can't afford or need integrated compression. This is typical in some boot procedures. You can have your initial ram disks in romfs format, and even if you don't use it after the boot, its code is still very small to waste memory on. This is also the single case when compression is easily achievable, as the initial ramdisk loader recognizes gzip compressed images. Currently no romfs code supports compression.
2. The other possible use when you need to use a Linux kernel on a device which has significant amount of read-only memory, and so it makes little sense to have a writable filesystem on it. This is the case with many embedded systems. uClinux is this case.
What the other file system:
There other file systems that are widely used in embedded linux system, such as ext2, ext3, cramfs(RAM based file system), JFFS2, NFS(we can even put root file system on a NFS server), and pseudo file systems such as proc, sysfs etc. They have different advantages and are chosen according to the specific applicaton.
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