Summer of Code results: Improving RAIDframe parity handling

The work to improve the parity handling in RAIDframe was done by Jed Davis as a 2009 Google Summer of Code project in NetBSD. The mentoring on this project was done by Greg Oster. This document summarizes the project and the results.

Goals

RAIDframe (the software RAID implementation in NetBSD) suffers from long parity checking/rebuilding times in the event of an unclean shutdown. The goal of this project was to implement a solution that greatly reduces the amount of time required to ensure that the parity is correct after an unclean shutdown.

Results

The main project goals were met. Jed's solution to the parity rebuilding problem is based on a "parity map". In this solution, the RAID set is divided into some number of "zones", where the parity status of each zone is reflected in the parity map. So while the existing RAIDframe code can be though of as having just a single zone (i.e. the entire RAID set) the new parity map code uses simple heuristics (e.g. minimum zone size of 25MiB per component, maximum of 4096 zones) to determine the number of zones and the size of the zones.

Part of the work involved exploring various zone sizes and investigating the performance implications of not only the zone sizes but the frequency of updating the parity map as well. There were also data consistency (e.g. order of write operations) and update issues (e.g. drive cache flushing) to deal with.

The code has yet to be merged into the main NetBSD tree, pending additional testing and verification of the code involved.