# Check reservation flexibility groups before matching different VM sizes

> DSE recommends recording each running VM’s exact size, its current flexibility group, and the applicable ratio before calculating coverage.

- Canonical URL: https://update.dsesecurity.com/updates/dse-20260917-033-check-reservation-flexibility-groups-before-matching-different-vm-sizes/
- Publisher: Detection Systems & Engineering (DSE Security)
- Author: DSE Security Editorial Team
- Published: 2026-09-17T16:34:27+00:00
- Modified: 2026-09-17T16:35:03+00:00
- Last reviewed by DSE: 2026-09-17
- Resource type: Guide
- DSE priority: Information
- Topics: IT
- Reading time: 1 minutes

## What you need to know

DSE recommends recording each running VM’s exact size, its current flexibility group, and the applicable ratio before calculating coverage.

## Potentially affected

This is a technical matching check for teams reviewing reservation coverage across Azure VM sizes, with commercial decisions handled by their normal approval process.

## DSE recommendation

DSE recommends recording each running VM’s exact size, its current flexibility group, and the applicable ratio before calculating coverage.

## Article

## Source facts

Microsoft says an instance-size-flexible reservation applies across sizes inside the same flexibility group, not across different groups. The relative ratios of the purchased size and running sizes determine how much VM usage the reservation covers. The guide identifies API or PowerShell access as the replacement for the retired flexibility-ratio CSV. [Microsoft Learn](https://learn.microsoft.com/en-us/azure/virtual-machines/reserved-vm-instance-size-flexibility)

## Applicability

This is a technical matching check for teams reviewing reservation coverage across Azure VM sizes, with commercial decisions handled by their normal approval process.

## DSE recommendation

DSE recommends recording each running VM’s exact size, its current flexibility group, and the applicable ratio before calculating coverage. Keep similarly named series separate until the group data confirms the relationship. Use the supported current data-access method and retain the retrieval date with the calculation. Have the resource owner reconcile the VM inventory before a financial reviewer considers purchase, exchange, or sizing decisions.

## Verification

Recalculate the proposed matching using the recorded ratios and compare it with observed reservation application where available. Flag any VM outside the selected group and any missing or stale ratio data. Do not report a coverage estimate as realized savings; preserve the assumptions and unresolved differences with the review.

## Official references

- [Microsoft: Virtual machine size flexibility -Azure Reserved VM Instances – Azure Virtual Machines | Microsoft Learn](https://learn.microsoft.com/en-us/azure/virtual-machines/reserved-vm-instance-size-flexibility)

## Primary reference

- Name: Virtual machine size flexibility -Azure Reserved VM Instances - Azure Virtual Machines | Microsoft Learn
- Authority: Microsoft Learn
- URL: https://learn.microsoft.com/en-us/azure/virtual-machines/reserved-vm-instance-size-flexibility
- Source publication date: Not stated by the source

## Citation and use

Preferred citation: “Check reservation flexibility groups before matching different VM sizes,” DSE Security, https://update.dsesecurity.com/updates/dse-20260917-033-check-reservation-flexibility-groups-before-matching-different-vm-sizes/
Publishing principles: https://update.dsesecurity.com/updates/dse-updates-editorial-methodology/
Usage and citation policy: https://update.dsesecurity.com/usage/
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