UNIX Filesystems: Evolution, Design, and Implementation

The BSD Memory-Based Filesystem (MFS)

The BSD team developed an unusual but interesting approach to memory-based filesystems as documented in [MCKU90]. Their goals were to improve upon the various RAM disk-based filesystems that had traditionally been used.

A RAM disk is typically a contiguous section of memory that has been set aside to emulate a disk slice. A RAM disk-based device driver is the interface between this area of memory and the rest of the kernel. Filesystems access the RAM disk just as they would any other physical device. The main difference is that the driver employs memory to memory copies rather than copying between memory and disk.

The paper describes the problems inherent with RAM disk-based filesystems. First of all, they occupy dedicated memory. A large RAM disk therefore locks down memory that could be used for other purposes. If many of the files in the RAM disk are not being used, this is particularly wasteful of memory. One of the other negative properties of RAM disks, which the BSD team did not initially attempt to solve, was the triple copies of data. When a file is read, it is copied from the file's location on the RAM disk into a buffer cache buffer and then out to the user's buffer. Although this is faster than accessing the data on disk, it is incredibly wasteful of memory.

The BSD MFS Architecture

Figure 11.3 shows the overall architecture of the BSD MFS filesystem. To create and mount the filesystem, the following steps are taken:

1. A call to ...