Azure's high availability includes a spectrum of options, allowing progressively more confidence that an application will stay alive in the face of failure.
Below are four options for ensuring a Virtual Machine (VM) is highly available. As we explain below, Azure’s high availability mechanisms, specifically Availability Zones, extend beyond VMs to additional Azure services.
Single VM
Running a Virtual Machine (VM) on Azure with no replication.
Availability Sets
Running a VM with one or more replicated copies on separate hardware within the same Availability Zone, providing resiliency against machine failure.
Availability Zones
Running a VM with one or more replicated copies in different availability zones provides resiliency against data center failure.
Region Pairs
Running a VM with one or more replicated copies on different Azure Regions (but always staying within the same geopolitical boundary, typically meaning the same country), protecting against natural disasters and large-scale outages.
Azure guarantees progressively higher uptime in its Service Level Agreements (SLA), depending on the availability option you choose:
90% uptime for on single-instance VMs with premium storage
95% uptime for VM deployed in an Availability Set (AS)
99% uptime for VM deployed across two Availability Zones (AZ)
An availability set is a concept within a data centre that is made up of multiple fault domains and update domains. When you create an availability set, you can specify how many fault domains and update domains you want in that availability set.
An Azure Availability Zone is a unique physical location within an Azure region. Each Availability Zone is made up of one or more data centres with independent power, cooling, and networking.
Difference between Azure Availability Set and Availability Zone
In short, the following are the two main differences between an Availability Set and an Availability Zone
An availability set protects your Azure resources from failures within data centres, whereas an availability zone protects from entire data centre failures.
From the Service Level Agreement (SLA) standpoint, i.e. uptime and connectivity - With Availability Set, Azure offers 99.95% SLA, whereas with Availability Zone, we have 99.99% SLA. So, there we go; with availability zones, Azure offers the industry's best 99.99% VM uptime SLA.
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