UNIX Filesystems: Evolution, Design, and Implementation

The Sun tmpfs Filesystem

Sun developed a memory-based filesystem that used the facilities offered by the virtual memory subsystem [SNYD90]. This differs from RAM disk-based filesystems in which the RAM disk simply mirrors a copy of a disk slice. The goal of the design was to increase performance for file reads and writes, allow dynamic resizing of the filesystem, and avoid an adverse effect on performance. To the user, the tmpfs filesystem looks like any other UNIX filesystem in that it provides full UNIX file semantics.

Chapter 7 described the SVR4 filesystem architecture on which tmpfs is based. In particular, the section An Overview of the SVR4 VM Subsystem in Chapter 7, described the SVR4/Solaris VM architecture. Familiarity with these sections is essential to understanding how tmpfs is implemented. Because tmpfs is heavily tied to the VM subsystem, it is not portable between different versions of UNIX. However, this does not preclude development of a similar filesystem on the other architectures.

Architecture of the tmpfs Filesystem

In SVR4, files accessed through the read() and write() system calls go through the seg_map kernel segment driver, which maintains a cache of recently accessed pages of file data. Memory-mapped files are backed by a seg_vn kernel segment that references the underlying vnode for the file. In the case where there is no backing file, the SVR4 kernel provides anonymous memory that is backed by swap space. This is described in the section Anonymous Memory ...