1994 Reports

Discovery and Hot Replacement of Replicated Read-Only File Systems, with Application to Mobile Computing

Zadok, Erez

We describe a mechanism for replacing files, including open files, of a read-only file system while the file system remains mounted; the act of replacement is transparent to the user. Such a “hot replacement“ mechanism can improve fault-tolerance, performance, or both. Our mechanism monitors, from the client side, the latency of operations directed at each file system. When latency degrades, the client automatically seeks a replacement file system that is equivalent to but hopefully faster than the current file system. The files in the replacement file system then take the place of those in the current file system. This work has particular relevance to mobile computers, which in some cases might move over a wide area. Wide area movement can be expected to lead to highly variable response time, and give rise to three sorts of problems: increased latency, increased failures, and decreased scalability. If a mobile client moves through regions having partial replicas of common file systems, then the mobile client can depend on our mechanism to provide increased fault tolerance and more uniform performance.