Vnode Layering Changes in NetBSD Version 1.5

In this paper, I describe changes to the vnode layer in NetBSD between versions 1.4 and 1.5 to assist layered filesystems. The major feature of these chano,es on the whole is that layered filesystems are now sufficiently robust for production applications. These changes divide readily into three groups: modifications to vnode locking along the lines suggested by Heidemann, changes to the vnode locking protocol, and the introduction of an overlay filesystem. In addition to describing these changes, I will describe the testing we have performed.