Allocate VM disks aligned to billing model

March 21, 2025 ยท View on GitHub

SYNOPSIS

Align to the Managed Disk billing increments to improve cost efficiency.

DESCRIPTION

Azure managed disks are billed based on predefined size increments. The billing increments are based on the disk storage type. These include:

  • Premium SSD - 4/ 8/ 16/ 32/ 64/ 128/ 256/ 512/ 1024/ 2048/ 4096/ 8192/ 16384/ 32768 GiB.
  • Standard SSD - 4/ 8/ 16/ 32/ 64/ 128/ 256/ 512/ 1024/ 2048/ 4096/ 8192/ 16384/ 32768 GiB.
  • Standard HDD - 32/ 64/ 128/ 256/ 512/ 1024/ 2048/ 4096/ 8192/ 16384/ 32768 GiB.
  • Ultra SSD - 4/ 8/ 16/ 32/ 64/ 128/ 256/ 512 GiB, then 1 TiB increments to 64 TiB.

If you provision a disk that is not aligned to the billing model, you will be billed for the next increment. For example, if a disk is provisioned at 33 GiB, the disk is billed as 64 GiB.

RECOMMENDATION

Consider aligning provisioned disk sizes to the billing increments for Managed Disks to improve cost efficiency.

EXAMPLES

Configure with Azure template

To deploy managed disks that pass this rule:

  • Set the properties.diskSizeGB property to a value that aligns to the billing model of the disk storage type. E.g. 32.

For example:

{
  "type": "Microsoft.Compute/disks",
  "apiVersion": "2023-04-02",
  "name": "[parameters('name')]",
  "location": "[parameters('location')]",
  "sku": {
    "name": "Premium_ZRS"
  },
  "properties": {
    "creationData": {
      "createOption": "Empty"
    },
    "diskSizeGB": 32
  }
}

Configure with Bicep

To deploy managed disks that pass this rule:

  • Set the properties.diskSizeGB property to a value that aligns to the billing model of the disk storage type. E.g. 32.

For example:

resource dataDisk 'Microsoft.Compute/disks@2023-04-02' = {
  name: name
  location: location
  sku: {
    name: 'Premium_ZRS'
  }
  properties: {
    creationData: {
      createOption: 'Empty'
    }
    diskSizeGB: 32
  }
}

NOTES

This rule has the following limitations:

  • This rule applies to managed disks using the following storage type: Ultra SSD, Premium SSD, and Standard SSD/ HDD disks.
    • Premium v2 disks are billed per provisioned disk capacity based on 1 GiB increments.
    • Unmanaged disks are ignored.
  • The rule does not fail if the disk size is within 5 GiB on the next size. For example: A 30 GiB disk is not aligned to the billed size of 32 GiB, but is within 5 GiB.
  • Disks with a marketplace purchase plan are ignored. These disks are predefined by the publisher are often unable to be reconfigured.