User file systems offer numerous advantages over their in-kernel implementations, such as ease of development and better system reliability. However, the heavy performance penalty incurred during their user is also well known. We observe that existing user file system frameworks are highly general: they implement a minimal interposition layer in the kernel that simply forwards all low-level requests to the user-space. While the design enables a number of usecases, it severely degrades performance due to frequent kernel-user context switching.
This work introduces ExtFUSE, a framework for developing extensible user file systems that also allows applications to register "thin" specialized request handlers in the kernel to meet their specific operative needs, while retaining the complex functionality in user-space.