Fault tolerant locking for shared disk filesystems

Abstract

Shared disk files ystems are crucial for providing direct, shared and high performance file system access on top of raw block based Storage Area Networks. One of the key components of a shared disk file system is a distributed lock manager which synchronizes direct concurrent access to data & metadata from multiple autonomous hosts. The lock manager should be fault tolerant for the entire system to be highly available. A design for a fault tolerant multicast based lock manager for the open source GFS shared disk file system is presented here. The GFS file system runs on Linux and currently lacks a fault tolerant distributed lock manager. The protocol presented here uses an efficient multicast based scheme to minimize the number of network hops involved in lock acquisition. Minimizing the number of network hops becomes very significant when dirty data associated with the lock is transferred through the interconnection network rather than doing disk writes. Ordered message and failure delivery needed for such a protocol was provided by the use pf a group communication toolkit. An existing group communication system (GCS) was ported to the kernel and used. The use of this communication system simplified the design and implementation of the protocol considerably. The GCS is expected to be useful in the design of other distributed protocols as well. The system was implemented and the performance tests were carried out on a Fibre channel based SAN setup. The performance was found to be as well as the current non fault tolerant system.