Summary

We have introduced the problem of crash consistency and discussed various approaches to attacking this problem. The older approach of building a file system checker works but is likely too slow to recover on modern systems. Thus, many file systems now use journaling. Journaling reduces recovery time from O(size-of-the-disk-volume) to O(size-of-the-log), thus speeding recovery substantially after a crash and restart. For this reason, many modern file systems use journaling. We have also seen that journaling can come in many different forms; the most commonly used is ordered metadata journaling, which reduces the amount of traffic to the journal while still preserving reasonable consistency guarantees for both file system metadata and user data. In the end, strong guarantees on user data are probably one of the most important things to provide; oddly enough, as recent research has shown, this area remains a work in progress“All File Systems Are Not Created Equal: On the Complexity of Crafting Crash-Consistent Applications” by Thanumalayan Sankaranarayana Pillai, Vijay Chidambaram, Ramnatthan Alagappan, Samer Al-Kiswany, Andrea C. Arpaci-Dusseau, Remzi H. Arpaci-Dusseau. OSDI ’14, Broomfield, Colorado, October 2014. A paper in which we study what file systems guarantee after crashes, and show that applications expect something different, leading to all sorts of interesting problems..