Guest - Virtual machine
A guest, in the context of mainframe virtual machines, refers to an operating system (such as z/OS, Linux on Z, or z/VSE) that runs within a virtual machine environment managed by a hypervisor like **z/VM**. Each guest operates as if it has exclusive access to its own dedicated hardware resources, even though these resources are virtualized and shared across multiple guests on a single physical mainframe.
Key Characteristics
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- Isolation: Each guest OS runs in its own isolated virtual machine, preventing interference between different operating systems or applications.
- Resource Virtualization: The z/VM hypervisor virtualizes physical resources (CPUs, memory, I/O devices) and presents them to each guest as dedicated virtual resources.
- Operating System Independence: A wide variety of operating systems can run as guests, including multiple instances of z/OS, Linux on Z, z/VSE, and even other z/VM instances.
- Virtual Machine Definition (VMDEF): Guests are defined in the z/VM directory, specifying their virtual hardware configuration (e.g., virtual CPUs, memory size, virtual devices).
- Virtual Console: Each guest typically has a virtual console for operator interaction, often accessible via
VTAMorTCP/IPthroughVM/CMS. - Dynamic Resource Allocation: Resources (CPU, memory) can often be dynamically reallocated to guests by the z/VM hypervisor based on workload demands.
Use Cases
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- Consolidation: Running multiple z/OS images, Linux on Z instances, or other operating systems on a single physical mainframe to reduce hardware footprint and power consumption.
- Development and Test Environments: Providing isolated environments for developing, testing, and staging applications (e.g., COBOL programs, CICS regions) without impacting production systems.
- Disaster Recovery: Creating standby guest systems that can be quickly activated in case of a failure in a primary system, offering rapid recovery capabilities.
- Software Evaluation: Safely evaluating new operating system versions, middleware (like DB2 or IMS), or application software in an isolated virtual environment.
- Workload Separation: Separating different types of workloads (e.g., production, development, batch processing) into distinct guest operating systems for better management, security, and resource control.
Related Concepts
A guest is intrinsically linked to the z/VM hypervisor, which creates and manages the virtual machine environment, providing the virtual hardware. While PR/SM (Processor Resource/Systems Manager) handles the initial hardware partitioning into LPARs (Logical Partitions), z/VM provides a further layer of virtualization, allowing many guests to share the resources of a single LPAR. Each guest operating system, such as z/OS, runs its own applications and subsystems (e.g., CICS, DB2, IMS, JCL batch jobs) within its virtualized environment, completely unaware that it is not running directly on physical hardware.
- Resource Sizing: Carefully size virtual CPUs, memory, and I/O resources for each guest based on its expected workload to ensure optimal performance and avoid resource contention.
- Monitoring and Tuning: Implement robust monitoring for both the z/VM hypervisor and individual guests to track resource utilization, performance, and identify potential bottlenecks, tuning as necessary.
- Security Configuration: Secure each guest OS independently, applying appropriate security patches, access controls (e.g., RACF), and network configurations, just as you would for a physical system.
- Virtual Device Optimization: Optimize virtual I/O device configurations (e.g., virtual disks, network adapters) for performance and reliability, leveraging z/VM's capabilities like
DASDsharing. - Regular Backups and DR Planning: Implement comprehensive backup and recovery strategies for all guest operating systems and their data, and integrate them into the overall disaster recovery plan.