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Azure VM Availability

 

Plan for maintenance and downtime

There are three scenarios that can lead to your virtual machine in Azure being impacted: unplanned hardware maintenance, unexpected downtime, and planned maintenance.

Unplanned Hardware Maintenance event occurs when the Azure platform predicts that the hardware or any platform component associated to a physical machine, is about to fail. When the platform predicts a failure, it will issue an unplanned hardware maintenance event. Azure uses Live Migration technology to migrate the Virtual Machines from the failing hardware to a healthy physical machine. Live Migration is a VM preserving operation that only pauses the Virtual Machine for a short time, but performance might be reduced before and/or after the event.

Unexpected Downtime is when the hardware or the physical infrastructure for the virtual machine fails unexpectedly. Unexpected downtime can include local network failures, local disk failures, or other rack level failures. When detected, the Azure platform automatically migrates (heals) your virtual machine to a healthy physical machine in the same datacenter. During the healing procedure, virtual machines experience downtime (reboot) and in some cases loss of the temporary drive.

Planned Maintenance events are periodic updates made by Microsoft to the underlying Azure platform to improve overall reliability, performance, and security of the platform infrastructure that your virtual machines run on. Most of these updates are performed without any impact upon your Virtual Machines or Cloud Services.

Setup availability sets

An Availability Set is a logical feature used to ensure that a group of related VMs are deployed so that they aren't all subject to a single point of failure and not all upgraded at the same time during a host operating system upgrade in the datacenter. VMs placed in an availability set should perform an identical set of functionalities and have the same software installed.

update and fault domains

Update Domains and Fault Domains helps Azure maintain high availability and fault tolerance when deploying and upgrading applications. Each virtual machine in an availability set is placed in one update domain and two fault domains.


Update domains

-- An update domain is a group of resources that can be updated and rebooted if required at the same time. 

-- Virtual machines get update domains automatically once they are put inside availability set. 

-- All virtual machines within that update domain will reboot together. 

-- Update domains are used for patching of the virtual machines. 

-- Only one update domain would be updated at the time

Fault domains

-- Fault domains define the group of virtual machines that share a common power source and network switch. 

-- Each and every fault domain contains some racks, and each rack contains virtual machine. Each of these Fault domain shares a power supply and a network switch. 

-- If there is a failure in the fault domain then all the resources in the fault domain become unavailable.

-- You should place your vms such a way that each fault domain gets one web server, one database server and like that 

Availability zones

Availability Zones is a high-availability offering that protects your applications and data from datacenter failures.

An Availability Zone in an Azure region is a combination of a fault domain and an update domain.



Considerations

  • Availability Zones are unique physical locations within an Azure region.
  • Each zone is made up of one or more datacenters equipped with independent power, cooling, and networking.
  • To ensure resiliency, there’s a minimum of three separate zones in all enabled regions.
  • The physical separation of Availability Zones within a region protects applications and data from datacenter failures.
  • Zone-redundant services replicate your applications and data across Availability Zones to protect from single-points-of-failure.
  • With Availability Zones, Azure offers industry best 99.99% VM uptime SLA.

vertical and horizontal scaling

Generally, there are two types of scaling: vertical scaling and horizontal scaling.

Vertical scaling

Vertical scaling, also known as scale up and scale down, means increasing or decreasing virtual machine sizes in response to a workload. Vertical scaling makes the virtual machines more (scale up) or less (scale down) powerful. Vertical scaling can be useful when:

  • A service built on virtual machines is under-utilized (for example at weekends). Reducing the virtual machine size can reduce monthly costs.
  • Increasing virtual machine size to cope with larger demand without creating additional virtual machines.

Horizontal scaling

Horizontal scaling, also referred to as scale out and scale in, where the number of VMs is altered depending on the workload. In this case, there is an increase (scale out) or decrease (scale in) in the number of virtual machine instances.

Scale set benefits:

  • All VM instances are created from the same base OS image and configuration. This approach lets you easily manage hundreds of VMs without additional configuration tasks or network management.
  • Scale sets support the use of the Azure load balancer for basic layer-4 traffic distribution, and Azure Application Gateway for more advanced layer-7 traffic distribution and SSL termination.
  • Scale sets are used to run multiple instances of your application. If one of these VM instances has a problem, customers continue to access your application through one of the other VM instances with minimal interruption.
  • Customer demand for your application may change throughout the day or week. To match customer demand, scale sets can automatically increase the number of VM instances as application demand increases, then reduce the number of VM instances as demand decreases. This is known as autoscale.
  • Scale sets support up to 1,000 VM instances. If you create and upload your own custom VM images, the limit is 600 VM instances.


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