Position Summary: The Conquest File System--Life after Disks

An-I Andy Wang , Peter Reiher , and Gerald J. Popek
UCLA, The Laboratory for Advanced Systems Research

Geoffrey H. Kuenning
Harvey Mudd College

The cost of paper and film has been a critical barrier to cross for a storage technology to achieve wide deployment and better economy of scale. By 2003, the declining cost of persistent RAM (e.g., battery-backed DRAM) will break this barrier, signifying the arrival of the persistent-RAM-based storage era.

Persistent RAM will not fully replace disks for many years. However, as RAM becomes cheap, memory can assume more roles of file systems. In particular, by 2005 high-end desktops can afford to be equipped with 4 to 10 Gbytes of persistent RAM for storage; this is sufficient to deliver nearly all aspects of file-system services, with the single exception of high-capacity storage.

The Conquest file system is designed to provide a transition from disk-based to persistent-RAM-based storage. Initially, we assume 2 to 4 Gbytes of persistent RAM and the popular single-user desktop environment. Unlike other memory file systems, Conquest can incrementally assume more responsibility for in-core storage as memory prices decline. The Conquest approach realizes most of the benefits of persistent-RAM-based file systems before persistent RAM becomes cheaply abundant. Conquest also benefits from the removal of disks as the primary storage by identifying disk-related complexities and isolating them from the critical path where possible.

Unlike cache, which treats main memory as a scarce resource, Conquest anticipates the abundance of cheap persistent RAM. Conquest uses disk to store only the data well suited for disk characteristics. Reducing the range of access patterns and characteristics anticipated by the file system translates into simpler disk optimizations.

Our initial Conquest implementation uses core memory to store all metadata, small files (currently based on a size threshold), executables, and dynamically linked libraries, leaving only the content of the large files on disk. All accesses to in-core data and metadata incur no data duplication or disk-related overhead, and executions are in-place. For the large-file-only disk storage, we can use a larger access granularity to reduce the seek-time overhead. Because most accesses to large files are sequential, we can relax many historical disk design constraints, such as complex layout heuristics intended to reduce fragmentation or average seek times.

Conquest also speeds up computing by allowing easy reuse of previously computed results. With an expanded API, Conquest allows direct storage of runtime data structures, states, or even processes that interact with the environment in constrained ways. Unlike memory-mapped files, storing runtime states under Conquest requires no compaction or alignment to page boundaries, which benefits many data representations. Direct storage of runtime states relieves developers of the need for serialization and deserialization. Applications can also take advantage of storing runtime data in the most appropriate form for processing. For example, network applications can store outbound data in the format of network packets to bypass both disk-to-memory and memory-to-network translations.

Storing data in core inevitably invites the question of reliability and data integrity. However, conventional techniques of sandboxing, access control, checkpointing, fsck, and object-oriented self-verification still apply. For example, Conquest still needs to perform frequent system backups. Conquest uses common memory protection mechanisms by having a dedicated memory address space for storage (assuming a 64-bit address space). A periodic fsck is still necessary, but it runs at memory speed. We are also exploring the object-store approach of having a typed memory area, so a pointer can be verified to be of a certain type before dereferencing.

Various areas of Conquest are under investigation. Memory under Conquest is a shared resource among execution, storage, and buffering for disk access. Finding the ?sweet spot? for system performance requires both modeling and empirical investigation. The ability for Conquest to store runtime states has the flavor of wide-address-space computing, which can be applied and extended to the distributed environment and database systems.

The Conquest prototype is operational under Linux 2.4.2. It is POSIX compliant and supports both in-core and on-disk storage. The source consists of 3,800 lines of kernel code, 1,400 lines of file-system-creation code, and 3,600 lines of testing code. Initial deployment and performance measurements are under way.


An-I Andy Wang, Geoffrey H. Kuenning, Peter Reiher, Gerald J. Popek. Position Summary: The Conquest File System--Life after Disks. Proceedings of the 8th IEEE Workshop on Hot Topics in Operating Systems (HotOS VIII), Schloss Elmau, Germany, May 2001.
< http://lasr.cs.ucla.edu/awang/papers/hotos2001.html>

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