xv6 reading homework

Throughout this semester, we expect that you will read the entire implementation of the xv6 kernel, accompanied by the xv6 book, which explains the rationale and design underlying the kernel's implementation. (The source code is also available as a Lions-style PDF)

For this lecture, read the kernel files kernel/log.c. You may find Chapter 10 (in particular, Sections 10.4-10.6) of the book useful in understanding the logging code.

To understand how the log is used, insert the following line of code before the log_write statement in the function writei() in kernel/fs.c:

printf("log write %d: %d %d %d\n", addr, bp->blockno, off, n);

Run make clean and then make qemu, and at the xv6 shell prompt run the following command:

$ echo hi > f

You should see output similiar to this:

log write 47: 47 368 16
log write 936: 936 0 2
log write 936: 936 2 1

The output indicates that log_write is called three times for this shell command. Why is log_write called 3 times? What does each log_write write? Does the first write above correspond to updating the root directory and the other two writes correspond to writing file f? (Feel free to add more print statements to the kernel to help you to answer the question.)

Submit your answer in an ASCII text file named homework.txt to the corresponding "Lecture N" assignment on Gradescope.

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