Vertical autoscale with Virtual Machine Scale sets

This article describes how to vertically scale Azure Virtual Machine Scale Sets with or without reprovisioning. For vertical scaling of VMs which are not in scale sets, refer to Vertically scale Azure virtual machine with Azure Automation.

Vertical scaling, also known as scale up and scale down, means increasing or decreasing virtual machine (VM) sizes in response to a workload. Compare this with horizontal scaling, also referred to as scale out and scale in, where the number of VMs is altered depending on the workload.

Reprovisioning means removing an existing VM and replacing it with a new one. When you increase or decrease the size of VMs in a VM Scale Set, in some cases you want to resize existing VMs and retain your data, while in other cases you need to deploy new VMs of the new size. This document covers both cases.

Vertical scaling can be useful when:

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

You can set up vertical scaling to be triggered based on metric based alerts from your VM Scale Set. When the alert is activated it fires a webhook that triggers a runbook which can scale your scale set up or down. Vertical scaling can be configured by following these steps:

  1. Create an Azure Automation account with run-as capability.
  2. Import Azure Automation Vertical Scale runbooks for VM Scale Sets into your subscription.
  3. Add a webhook to your runbook.
  4. Add an alert to your VM Scale Set using a webhook notification.

Note

Vertical autoscaling can only take place within certain ranges of VM sizes. Compare the specifications of each size before deciding to scale from one to another (higher number does not always indicate bigger VM size). You can choose to scale between the following pairs of sizes:

VM sizes scaling pair
Standard_A0 Standard_A11
Standard_D1 Standard_D14
Standard_DS1 Standard_DS14
Standard_D1v2 Standard_D15v2
Standard_G1 Standard_G5
Standard_GS1 Standard_GS5

Create an Azure Automation Account with run-as capability

The first thing you need to do is create an Azure Automation account that will host the runbooks used to scale the VM Scale Set instances. Recently Azure Automation introduced the "Run As account" feature which makes setting up the Service Principal for automatically running the runbooks on a user's behalf very easy. You can read more about this in the article below:

Import Azure Automation Vertical Scale runbooks into your subscription

The runbooks needed to vertically scale your VM Scale Sets are already published in the Azure Automation Runbook Gallery. To import them into your subscription follow the steps in this article:

Choose the Browse Gallery option from the Runbooks menu:

Runbooks to be imported

The runbooks that need to be imported are shown. Select the runbook based on whether you want vertical scaling with or without reprovisioning:

Runbooks gallery

Add a webhook to your runbook

Once you've imported the runbooks you'll need to add a webhook to the runbook so it can be triggered by an alert from a VM Scale Set. The details of creating a webhook for your Runbook are described in this article:

Note

Make sure you copy the webhook URI before closing the webhook dialog as you will need this in the next section.

Add an alert to your VM Scale Set

Below is a PowerShell script which shows how to add an alert to a VM Scale Set. Refer to the following article to get the name of the metric to fire the alert on: Azure Monitor autoscaling common metrics.

$actionEmail = New-AzureRmAlertRuleEmail -CustomEmail user@contoso.com
$actionWebhook = New-AzureRmAlertRuleWebhook -ServiceUri <uri-of-the-webhook>
$threshold = <value-of-the-threshold>
$rg = <resource-group-name>
$id = <resource-id-to-add-the-alert-to>
$location = <location-of-the-resource>
$alertName = <name-of-the-resource>
$metricName = <metric-to-fire-the-alert-on>
$timeWindow = <time-window-in-hh:mm:ss-format>
$condition = <condition-for-the-threshold> # Other valid values are LessThanOrEqual, GreaterThan, GreaterThanOrEqual
$description = <description-for-the-alert>

Add-AzureRmMetricAlertRule  -Name  $alertName `
                            -Location  $location `
                            -ResourceGroup $rg `
                            -TargetResourceId $id `
                            -MetricName $metricName `
                            -Operator  $condition `
                            -Threshold $threshold `
                            -WindowSize  $timeWindow `
                            -TimeAggregationOperator Average `
                            -Actions $actionEmail, $actionWebhook `
                            -Description $description

Note

It is recommended to configure a reasonable time window for the alert in order to avoid triggering vertical scaling, and any associated service interruption, too often. Consider a window of least 20-30 minutes or more. Consider horizontal scaling if you need to avoid any interruption.

For more information on how to create alerts refer to the following articles:

Summary

This article showed simple vertical scaling examples. With these building blocks - Automation account, runbooks, webhooks, alerts - you can connect a rich variety of events with a customized set of actions.