Technical Description of the DECsafe Available Server Environment

such as the small computer system interface (SCSI) in the Digital UNIX operating system provided the opportunity to make existing disk-based services more available. Since high availability is an important feature to mission-critical applications such as database and file system servers, we started to explore high-availability solutions for the UNIX operating system environment. The outcome of this effort is the DECsafe Available Server Environment (ASE), an integrated organization of computer systems and external disks connected to one or more shared SCSI buses. In the first section of this paper, we review the many product requirements that needed to be explored. We then define the ASE concepts. In the next section, we discuss the design of the ASE components. In subsequent sections, we describe some of the issues that needed to be overcome during the product’s design and development: relocating client-server applications, event monitoring and notification, network partitioning, and management of available services. Further, we explain how ASE deals with problems concerning multihost SCSI; the cross-organizational logistical issues of developing specialized SCSI hardware and firmware features on high-volume, low-priced standard commodity hardware; and modifications to the Network File System (NFS) to be both highly available and backward compatible.