Cramfs

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The compressed ROM/RAM file system (or cramfs) is a free (GPL'ed) read-only Linux file system designed for simplicity and space-efficiency. It is mainly used in embedded and small-footprint systems.

Unlike a compressed image of a conventional file system, a cramfs image can be used as it is, i.e. without first decompressing it. For this reason, some Linux distributions use cramfs for initrd images (Debian 3.1 in particular) and installation images (SUSE Linux in particular), where there are constraints on memory and image size.

In 2013, Linux maintainers indicated that cramfs was made obsolete by squashfs,^[1] but the file system got rehabilitated in 2017 for use in low-memory devices where using squashfs may not be viable.^[2]