UNIX Filesystems: Evolution, Design, and Implementation

Installing Debugging Support

Analyzing the filesystem source code is one way to learn about how filesystems work. However, it is extremely difficult following this method to truly understand the flow through the kernel and filesystem in response to certain operations. There is no better method than installing and using one of the different kernel debuggers allowing you to stop in specific functions, display stack backtraces and function arguments, and print other useful information.

There are three main methods under which a filesystem or indeed any other part of the kernel can be debugged. The first approach involves using the kernel printk() command which is very similar to printf(). The second approach involves using a standalone debugger such as kdb whereby flow can be stopped by placing explicit breakpoints or by entering a special key sequence to enter the debugger. The third approach involves the use of two machines connected through a serial cable and over which gdb can be used for source level debugging.

The following sections describe each of these approaches. The amount of work to perform each task is considerably different with printk() being the simplest approach while the gdb approach involves more time to set up and an additional machine. For readers who wish to experiment and have access to all the available resources it is recommended that you start with printk() first, then move to kdb, and finally to gdb.

The following sections assume some familiarity with debugging ...