(Illustration by Gaich Muramatsu)
(Illustration by Gaich Muramatsu)
RVM, a Recoverable Virtual Memory system, is a mechanism to support persistent virtual memory in the face of system crashes. The current state of affairs is that software rather than hardware is limiting factor is systems reliability ], and so RVM is intended to assist in building more robust software. RVMs principal reliability criterion is to provide permanence of committed changes and integrity of data over operating system crashes. The fatal nature of these all-too-frequent events often leaves data structures in a shambles. But the all-or-none property of atomic transactions can insure consistency of data structures over crashes and makes the application significantly more robust with small cost. Using simple, non-nested local transactions, it provides a way to automatically checkpoint the state of memory before an operation and restore that state in the event of a crash.
RVM can also provide some resilience in the face of localized media failures, but massive media failures such as head crashes are not recoverable since storage is not mirrored. Applications must rely on periodic backups, or other mechanisms, to recover from these rare events. While mirroring isnt supported, RVM does not preclude it: applications can provide device drivers for storage media that mirror data without RVMs knowledge.
Because RVM is not a server, but a tool for building servers, there are important differences between RVM and database transaction systems. RVM does not provide application level synchronization or serialization functions, although RVM synchronizes its internal actions so that applications can use multiple threads. Also, RVM manages storage as an unstructured address space; no object abstractions are supported.
None of these responsibilities are necessary for simple transactions, although some, or all, will be needed for a given application. RVMs design is restricted to only the minimum functionality necessary to support the restoration mechanisms needed for exception handling in a persistent virtual memory. Other services can be provided by libraries and servers that use RVM, or by application-specific functions.
Much of the motivation for building RVM has come from experience with Camelot ], a large, full-featured transaction system. While Camelot is much more capable than RVM, it was discovered in building CODA ], a high-availability file system, that most of the benefit of transactions was gained by using only a fraction of Camelots facilities. RVM is an experiment to discover the minimum transaction processing functionality useful in building complex applications such as file servers and other non-database systems.
The decision to use RVM rather than a more complex transaction system can be made by considering the underlying design assumptions. If the following constraints are not too restrictive for the application, RVM will be a good choice:
These assumptions greatly simplify a transaction system and reflect RVMs design and implementation philosophy that stresses simplicity and ease of use. To make RVM highly portable, the implementation uses only Unix features that are standard across a wide variety of Unix and Mach/Unix implementations. Mach-specific extensions are used only for optional features and performance enhancements. Optional features, when provided, exact no penalty when not chosen; at worst, one will not receive the benefits of performance enhancing options.
While RVM directly supports only a single server at a single site, an application can provide the communication mechanisms to integrate multiple instantiations of RVM into a widely distributed system. RVMs first application, CODA, will use this technique to manage multiple file servers.
RVM is inoffensive and inconspicuous in action: it does not complicate or preclude the use of standard tools such as profilers and debuggers. Recognizing that the disk operations inherent in transaction processing are expensive, RVM allows the application considerable freedom to manage such operations.