Regions and availability - VMs

Overview

You create Azure resources in defined geographic regions. Within each region, multiple datacenters exist to provide for redundancy and availability.

Region pairs

Each Azure region is paired with another region within the same geography (such as US, Europe, or Asia). This approach allows for the replication of resources, such as VM storage, across a geography that should reduce the likelihood of natural disasters, civil unrest, power outages, or physical network outages affecting both regions at once. Additional advantages of region pairs include:

  • In the event of a wider Azure outage, one region is prioritized out of every pair to help reduce the time to restore for applications.
  • Planned Azure updates are rolled out to paired regions one at a time to minimize downtime and risk of application outage.
  • Data continues to reside within the same geography as its pair (except for Brazil South) for tax and law enforcement jurisdiction purposes.

Storage availability

Azure Managed Disks

  • Locally redundant storage (LRS)
    • Replicates your data three times within the region in which you created your storage account.

Premium Managed Disks are backed by Solid-State Drives (SSDs) and Standard Managed Disks are backed by regular spinning disks. Both Premium and Standard Managed Disks are charged based on the provisioned capacity for the disk.

Storage account-based disks

  • Locally redundant storage (LRS)
    • Replicates your data three times within the region in which you created your storage account.
  • Zone redundant storage (ZRS)
    • Replicates your data three times across two to three facilities, either within a single region or across two regions.
  • Geo-redundant storage (GRS)
    • Replicates your data to a secondary region that is hundreds of miles away from the primary region.
  • Read-access geo-redundant storage (RA-GRS)
    • Replicates your data to a secondary region, as with GRS, but also then provides read-only access to the data in the secondary location.
  • Premium storage is backed by Solid-State Drives (SSDs) and is charged based on the capacity of the disk.
  • Standard storage is backed by regular spinning disks and is charged based on the in-use capacity and desired storage availability.
    • For RA-GRS, there is an additional Geo-Replication Data Transfer charge for the bandwidth of replicating that data to another Azure region.

Availability Sets

An availability set is a logical grouping of VMs within a datacenter that allows Azure to understand how your application is built to provide for redundancy and availability.

An availability set is composed of two additional groupings that protect against hardware failures and allow updates to safely be applied - fault domains (FDs) and update domains (UDs).

Fault domains

common power source and network switch

A fault domain is a logical group of underlying hardware that share a common power source and network switch, similar to a rack within an on-premises datacenter. As you create VMs within an availability set, the Azure platform automatically distributes your VMs across these fault domains. This approach limits the impact of potential physical hardware failures, network outages, or power interruptions.

Update domains

underlying hardware rebooted at the same time

An update domain is a logical group of underlying hardware that can undergo maintenance or be rebooted at the same time. As you create VMs within an availability set, the Azure platform automatically distributes your VMs across these update domains. This approach ensures that at least one instance of your application always remains running as the Azure platform undergoes periodic maintenance. The order of update domains being rebooted may not proceed sequentially during planned maintenance, but only one update domain is rebooted at a time.

Managed Disk fault domains

For VMs using Azure Managed Disks, VMs are aligned with managed disk fault domains when using a managed availability set. This alignment ensures that all the managed disks attached to a VM are within the same managed disk fault domain. Only VMs with managed disks can be created in a managed availability set.

Availability zones

Availability zones, an alternative to availability sets, expand the level of control you have to maintain the availability of the applications and data on your VMs. An Availability Zone is a physically separate zone within an Azure region. There are three Availability Zones per supported Azure region. Each Availability Zone has a distinct power source, network, and cooling, and is logically separate from the other Availability Zones within the Azure region. By architecting your solutions to use replicated VMs in zones, you can protect your apps and data from the loss of a datacenter. If one zone is compromised, then replicated apps and data are instantly available in another zone.

Compute