Azure Reserved Instance size flexibility

Mit den inzwischen weit verbreiteten Cloud-Services auf dem Markt, hat sich die Flexibilität für Kunden vervielfacht. Schnell eine virtuelle Testumgebung per Knopfdruck auf Azure starten und anschliessend wieder bereinigen. Möchte man aber durchgehend mehrere virtuelle Maschinen auf Azure betreiben, so lohnt es sich dies zu planen und mittels einer Reservierung kosten zu sparen. Um die Flexibilität nicht komplett einbüssen zu müssen, gibt es das «Azure Reserved Instance size flexibility» Angebot, welches für VMs und Scale Sets zum Einsatz kommt. In diesem Artikel möchte ich näher darauf eingehen, wie «Azure Reserved Instance size flexibility» funktioniert und du es zu deinem Gunsten einsetzen und Geld sparen kannst.

What is Azure Reserved Instance size flexibility?

When purchasing a VM Reserved Instance, you have the option to set the reservation to "Capacity priority" or "Instance size flexibility". By default, the option "Instance size flexibility" is already activated, which I will discuss in more detail here.

Azure Reserved Instance size flexibility is simply getting or buying a reservation ratio. Depending on the size of a reservation, you get more or less ratio points. These ratio points are claimed by the VMs that qualify for them. It is important to note that the ratio points are only valid within a certain VM type group, the so-called "Instance Size Flexibility Group". The following image shows the group "DSv3 Series" as an example. On the one hand it shows how many ratio points a reservation is worth and on the other hand how many ratio points a VM consumes.

So in this example, a DSv3 reservation gets the corresponding number of ratio points, which can then be claimed by any DSv3 VMs.
You can download the complete list at this Link .

What does Azure Reserved Instance size flexibility do for me?

Among other things, Azure attracts attention with its flexibility, as already mentioned at the beginning of this article. If you reserve a VM, this flexibility is partly lost. But with the instance size flexibility, you can regain flexibility and still benefit financially from the reservation.

Example how size flexibility works

The company "Graber IT" creates a new service based on Azure VMs. Since the company cannot yet estimate which VM is the right one and how fast and strongly the service will scale, it starts a B2s and a D2s_v3 machine. To save costs, Graber IT reserves the two VMs for 3 years.

The newly created service is well received on the market, which is why the D2s_v3 VM no longer meets the growing requirements after just one year. Graber IT decides to scale the VM to D4s_V3. Although the company "only" has a D2s_v3 reservation, it is still credited proportionally. Note: Although the company also has a D2s_v4 and D4s_v4 reservation (see image > Reservation Pot), they are not credited because they are not in the same "Instance Size Flexibility Group" (v3 vs v4).

Concretely, this means the following. Graber IT has a D2s_v3 reservation which is worth 1 ration point. But the VM scaled to a D4s_v3 now consumes its 2 ration points. The company now has two options.

1. Graber IT makes another reservation for a D2s_v3 VM and receives another ratio point. Thus the consumption of 2 ration points of the D4s_v3 VM would be saturated.
2. Graber IT makes NO further reservation. Thus, 50% of the compute costs of the D4s_v3 will be charged monthly as usual after Consumption, since only 50% of the necessary ratio points are covered.

Conclusion - Use size flexibility in your favor

Are you launching new VMs on Azure that will last for at least a year or more? So don't wait with the reservation and do it right away. Thanks to the instance size flexibility, you can continue to use them even if you need to scale the VM size. Start small, grow fast!

Sources:

https://docs.microsoft.com/en-us/azure/virtual-machines/reserved-vm-instance-size-flexibility?WT.mc_id=AZ-MVP-5004129

https://docs.microsoft.com/en-us/azure/cost-management-billing/reservations/manage-reserved-vm-instance?WT.mc_id=AZ-MVP-5004129#change-optimize-setting-for-reserved-vm-instances

https://docs.microsoft.com/en-us/azure/virtual-machines/reserved-vm-instance-size-flexibility?WT.mc_id=AZ-MVP-5004129#instance-size-flexibility-ratio-for-vms