Hotspot Question You plan to deploy a critical line-of-business application to Azure. The application will run on an Azure virtual machine. You need to recommend a deployment solution for the applicat

Question

Exhibit

Answer Area

• Minimum number of virtual machines:1
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• Minimum number of availability zones:1
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Explanation

To achieve a 99.99% availability SLA for an application running on Azure Virtual Machines, the minimum recommended deployment is two virtual machines distributed across two distinct Azure Availability Zones.

Approach. The core requirement is to achieve a guaranteed availability of 99.99 percent for a critical line-of-business application running on an Azure virtual machine. According to the Azure Virtual Machine Service Level Agreement (SLA):

1. 99.9% SLA: A single Azure virtual machine with Premium SSDs (or Standard SSDs/HDDs at 99.5%) offers an SLA of 99.9%. This is insufficient for the 99.99% requirement.
2. 99.95% SLA: Deploying two or more VMs within an Availability Set (which are typically within a single data center/zone) provides a 99.95% SLA. This is also insufficient.
3. 99.99% SLA: To achieve 99.99% availability for Virtual Machines, Azure requires deploying two or more instances of your Virtual Machines across two or more Availability Zones within the same Azure region. Each Availability Zone is a physically separate location within an Azure region, with independent power, cooling, and networking.

Therefore, to meet the 99.99% SLA while adhering to the 'minimum number' requirement:

• The 'minimum number of virtual machines' must be 2 (as 'two or more instances' are needed).
• The 'minimum number of availability zones' must be 2 (as 'across two or more Availability Zones' are needed).

Selecting '2' for both options provides the necessary redundancy and geographical isolation to meet the stringent 99.99% availability target with the lowest resource overhead.

Common mistakes.

• common_mistake. 1. Selecting 1 Virtual Machine: A single VM, regardless of where it's deployed, does not provide the redundancy needed for 99.99% availability. A single VM, even with Premium SSD, only offers 99.9% availability, which is lower than the required 99.99%. If that single VM experiences an outage, the application becomes unavailable.

1. Selecting 1 Availability Zone: Deploying multiple VMs within a single Availability Zone (e.g., using an Availability Set within that zone) provides a 99.95% SLA. While this offers protection against hardware failures within that zone, it does not protect against a complete outage of the entire Availability Zone itself. The requirement for 99.99% explicitly necessitates distribution across multiple zones.
2. Selecting 3 Virtual Machines or 3 Availability Zones: While deploying 3 VMs across 3 Availability Zones would certainly meet the 99.99% SLA, the question specifically asks for the minimum number. Deploying more resources than necessary would lead to higher costs without providing additional SLA benefit beyond the 99.99% already achieved with 2 VMs in 2 AZs. Hence, these choices are not 'minimum'.

Concept tested. Azure Virtual Machine Service Level Agreements (SLAs), High Availability design principles, Azure Availability Zones, and the cost-effectiveness of selecting minimum resources to meet specific availability targets.

Reference. https://azure.microsoft.com/en-us/support/legal/sla/virtual-machines/v1_9/

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