Volume III, Issue IV, April 2014 IJLTEMAS ISSN 2278 - 2540 An Introduction to Single System Image (SSI) Cluster Technique Tarun Kumawat [CSE] , JECRC UDML College of Engineering. Kukas, Jaipur, Rajasthan, India1 Sandeep Tomar [CSE] , Arya College of Engineering & I.T. Kukas, Jaipur, Rajasthan, India2 Mohit Gupta [CSE] , Arya College of Engineering & I.T. Kukas, Jaipur, Rajasthan, India3 [email protected] [email protected] 3 [email protected] beowulf.myinstitute.edu), although the cluster Abstract-Cluster computing is not a new area of computing. may have multiple physical host nodes to serve It is, however, evident that there is a growing interest in its the login session. The system transparently usage in all areas where applications have traditionally used distributes user’s connection requests to different parallel or distributed computing platforms. A Single System physical hosts to balance load. Image (SSI) is the property of a system that hides the Single user interface: The user should be able to heterogeneous and distributed nature of the available use the cluster through a single GUI. The resources and presents them to users and applications as a single unified computing resource. SSI can be enabled in interface must have the same look and feel than numerous ways, this range from those provided by extended the one available for workstations (e.g., Solaris hardware through to various software mechanisms. SSI OpenWin or Windows NT GUI). means that users have a globalised view of the resources Single process space: All user processes, no available to them irrespective of the node to which they are matter on which nodes they reside, have a unique physically associated. cluster-wide process id. A process on any node can create child processes on the same or Keywords: Cluster SSI, SCO UnixWare, GLUnix, MOSIX different node (through a UNIX fork). A process should also be able to communicate with any I. INTRODUCTION other process (through signals and pipes) on a remote node. Clusters should support globalised Single System Image (SSI) is the property of a process management and allow the management A system that hides the heterogeneous and distributed and control of processes as if they are running on nature of the available resources and presents them to local machines. users and applications as a single unified computing Single memory space: Users have an illusion of a resource. SSI can be enabled in numerous ways, this big, centralised main memory, which in reality range from those provided by extended hardware through may be a set of distributed local memories. to various software mechanisms. SSI means that users Software DSM approach has already been used have a globalised view of the resources available to them to achieve single memory space on clusters. irrespective of the node to which they are physically Another approach is to let the compiler distribute associated. Furthermore, SSI can ensure that a system the data structure of an application across continues to operate after some failure (high availability) multiple nodes. It is still a challenging task to as well as ensuring that the system is evenly loaded and develop a single memory scheme that is efficient, providing communal multiprocessing (resource platform independent, and able to support management and scheduling). sequential binary codes. SSI design goals for cluster-based systems are mainly Single I/O space (SIOS): This allows any node to focused on complete transparency of resource perform I/O operations on local or remotely management, scalable performance, and system located peripheral or disk device. In this SIOS availability in supporting user applications [1][2][3][5][7]. design, disks associated to cluster nodes, A SSI can be defined as the illusion [1][2], created by network-attached RAIDs, and peripheral devices hardware or software, that presents a collection of form a single address space. resources as one, more powerful unified resource. Single file hierarchy: On entering into the system, the user sees a single, huge file-system II. SERVICES AND BENEFITS image as a single hierarchy of files and directories under the same root directory that The key services of a single-system image cluster include transparently integrates local and global disks the following [1][3][4]: and other file devices. Examples of single file Single entry point: A user can connect to the hierarchy include NFS, AFS, xFS, and Solaris cluster as a virtual host (like telnet MC Proxy. www.ijltemas.in Page 207 Volume III, Issue IV, April 2014 IJLTEMAS ISSN 2278 - 2540 Single virtual networking: This means that any It offers the same command syntax as in other node can access any network connection systems and thus reduces the risk of operator throughout the cluster domain even if the errors, with the result that end-users see an network is not physically connect to all nodes in improved performance, reliability and higher the cluster. Multiple networks support a single availability of the system. virtual network operation. It allows to centralise/decentralise system Single job-management system: Under a global management and control to avoid the need of job scheduler, a user job can be submitted from skilled administrators for system administration. any node to request any number of host nodes to It greatly simplifies system management and thus execute it. Jobs can be scheduled to run in either reduced cost of ownership. batch, interactive, or parallel modes. Examples of It provides location-independent message job management systems for clusters include communication. GLUnix, LSF, and CODINE. It benefits the system programmers to reduce the Single control point and management: The entire time, effort and knowledge required to perform cluster and each individual node can be task, and allows current staff to handle larger or configured, monitored, tested and controlled more complex systems. from a single window using single GUI tools, It promotes the development of standard tools much like an NT workstation managed by the and utilities. Task Manger tool. Checkpointing and Process Migration: III. SSI LAYERS/LEVELS Checkpointing is a software mechanism to periodically save the process state and The two important characteristics of SSI [1][2] are: intermediate computing results in memory or 1. Every SSI has a boundary, disks. This allows the roll back recovery after a 2. SSI support can exist at different levels within a system failure. Process migration is needed in dynamic — one able to be built on another. load balancing among the cluster nodes and in supporting Checkpointing. Figure 1 shows the SSI can be implemented in one or more of the following functional relationships among various key levels: middleware packages. Hardware, Operating System (so called underware [5]), These middleware packages are used as interfaces Middleware (runtime subsystems), between user applications and cluster hardware and OS Application. platforms. They support each other at the management, programming, and implementation levels. A good SSI is usually obtained by a co-operation between all these levels as a lower level can simplify the implementation of a higher one. A. Hardware Level Systems such as Digital/Compaq Memory Channel [8] and hardware Distributed Shared Memory (DSM) [8] offer SSI at hardware level and allow the user to view a cluster as a shared-memory system. Digital's Memory Channel is designed to provide a reliable, powerful and efficient clustering interconnect. It provides a portion of global virtual shared memory by mapping portions of Figure 1. The relationship between middleware modules [3]. remote physical memory as local virtual memory (called reflective memory). The most important benefits of SSI include the following Memory Channel consists of two components: a PCI [1]: adapter and a hub. Adapters can also be connected It provides a simple, straightforward view of all directly to another adapter without using a hub. The host system resources and activities, from any node in interfaces exchange heartbeat signals and implement flow the cluster. control timeouts to detect node failure or blocked data It frees the end-user from having to know where transfers. The link layer provides error detection through a in the cluster an application will run. 32 bit CRC generated and checked in hardware. Memory It allows the use of resources in a transparent Channel uses point-to-point, full-duplex switched 8x8 way irrespective of their physical location. crossbar implementation. It lets the user work with familiar interface and commands and allows the administrator to To enable communication over the Memory Channel manage the entire cluster as a single entity. network, applications map pages as read- or write-only into their virtual address space. Each host interface www.ijltemas.in Page 208 Volume III, Issue IV, April 2014 IJLTEMAS ISSN 2278 - 2540 contains two page control tables (PCT), one for write and 1) SCO UnixWare one for read mappings. For read-only pages, a page is pinned down in local physical memory. Several page UnixWare NonStop Clusters is SCO's high availability attributes can be specified: receive enable, interrupt on software. It significantly broadens hardware support receive, remote read etc. If a page is mapped as write- making it easier and less expensive to deploy the most only, a page table entry is created for an appropriate page advanced clustering software for Intel systems. It is an in the interface 128 Mbytes of PCI address space. Page extension to the UnixWare operating system where all attributes can be used to store a local copy of each packet, applications run better and more reliably inside a Single request acknowledgement message from receiver side for System Image (SSI) environment that removes the each packet, and define the packets as broadcast or point- management burden. It features standard IP as the to-point packets. Broadcasts are forwarded to each node interconnect, removing the need for any proprietary attached to the network.