Assessing Redundancy Strategies to Improve Availability in Virtualized System Architectures

TL;DR

The paper tackles the problem of ensuring high availability for a Nextcloud-based file server deployed in a private cloud managed by Apache CloudStack. It applies Stochastic Petri Nets (SPNs) to model and compare four redundancy configurations (baseline, host-level, VM-level, and combined host+VM) and demonstrates that combined redundancy delivers near-100% availability with minimal downtime. Key contributions include SPN-based availability models for private cloud setups, the introduction of cold-standby scenarios, and a case study with quantitative results showing the practical impact of redundancy strategies on dependability. The work provides a methodological framework to guide private-cloud design decisions and supports engineers in planning resilient infrastructures. Future directions include exploring the effect of VM power capacities and hybrid cloud scenarios on availability.

Abstract

Cloud-based storage platforms are becoming more common in both academic and business settings due to their flexible access to data and support for collaborative functionalities. As reliability becomes a vital requirement, particularly for organizations looking for alternatives to public cloud services, assessing the dependability of these systems is crucial. This paper presents a methodology for analyzing the availability of a file server (Nextcloud) hosted in a private cloud environment using Apache CloudStack. The analysis is based on a modeling approach through Stochastic Petri Nets (SPNs) that allows the evaluation of different redundancy strategies to enhance the availability of such systems. Four architectural configurations were modeled, including the baseline, host-level redundancy, virtual machine (VM) redundancy, and a combination of both. The results show that redundancy at both the host and VM levels significantly improves availability and reduces expected downtime. The proposed approach provides a method to evaluate the availability of a private cloud and support infrastructure design decisions.

Figures (7)

• Figure 1: Example of a stochastic Petri net.
• Figure 2: The adopted methodology.
• Figure 3: Test environment architecture.
• Figure 4: Proposed availability model.
• Figure 5: Architecture considering cold standby for both the virtual machine and the host.