Designing a Fast On-line Backup System for a Log-structured File System

lost through user error, software or media failure, or site disaster but are unwilling to devote system resources or downtime to make backup copies of the data. Furthermore, with the rapid growth in the use of data storage and the tendency to move systems toward complete utilization (i.e., 24-hour by 7-day operation), the practice of taking the system off line to back up data is no longer feasible. The Spiralog file system, an optional component of the OpenVMS Alpha operating system, incorporates a new approach to the backup process (called simply backup), resulting in a number of substantial customer benefits. By exploiting the features of logstructured storage, the backup system combines the advantages of two different traditional approaches to performing backup: the flexibility of file-based backup and the high performance of physically oriented backup. The design goal for the Spiralog backup system was to provide customers with a fast, application-consistent, on-line backup. In this paper, we explain the features of the Spiralog file system that helped achieve this goal and outline the design of the major backup functions, namely volume save, volume restore, file restore, and incremental management. We then present some performance results arrived at using Spiralog version 1.1. The paper concludes with a discussion of other design approaches and areas for future work.