LOCUS a network transparent, high reliability distributed system

Abstract

LOCUS is a distributed operating system that provides a very high degree of network transparency while at the same lime supporting lligh performance and automatic replication of storage, By network transparency we mean thai at the syslem call interface there is no need to mention anything network related. Knowledge of the network and code to inleracl with foreign sites is below this interface and is thus hidden from both users and programs under norm~ll conditions. LOCUS is application code compatible with Unix 2, and performance compares favorably with standard, single syslem Unix. LOCUS runs on a high bandwidth, low delay local network. It is designed to permil both a significant degree of local autonomy for each site in the network while still providing a network-wide, location independent name struclure. Atomic file operations and extensive synchronization are supported.Small, slow sites without local mass slore can coexist in the same network with much larger and more powerful machines without larger machines being slowed down through forced interaction with slower ones. Graceful operation during network topology changes is supported.

Improving storage system availability with D-GRAID

“…For example, Walker et al discussed the importance of directory namespace replication within the Locus distributed system [Popek et al 1981]. The Coda mobile file system also takes explicit care with regard to the directory tree [Kistler and Satyanarayanan 1992].…”

“…For time overheads, a higher degree of replication implies lowered write performance for naming and system meta-data operations. However, others have observed that there is a lack of update activity at higher levels in the directory tree [Popek et al 1981], and lazy update propagation can be employed to reduce costs [Savage and Wilkes 1996]. …”

A weighted voting algorithm for replicated directories

“…Techniques for alleviating this problem have been developed. For example, each file open operation in the Locus distributed file system ensures the currency of data by consulting a known synchronization site [23]. Locus maintains availability after synchronization site failure by nominating a new synchronization site.…”

Object identity

“…In some distributed systems, such as the Cambridge File Server [Dion 1980] and the LOCUS system [Popek et al 1981], the identifiers of files (the objects of the systems) are structured, where pan of the structure captures an aspect of the location of the object, such as a disk or server. For example, in LOCUS part of the identifier of a file identifies a (logical) volume where the file is located.…”