Reservations for VMs and ratios. What VM size should I purchase reservation for?

Question

I want to purchase reservations for virtual machines and I am not sure how to decide what to purchase in regards to the different VM sizes. If I have one B4ms VM (with ratio 8), my understanding is that I for instance can choose to purchase one reservation for B4ms or purchase 8 reservations for B1ms (with ratio 1). What are the pros and cons for the different options? Should I purchase multiple reservations for a smaller size or one reservations for the large size? What is best practice?

Answer 1

Instance Size Flexibility ratio comes into the picture when you have multiple VMs in the same VM family. However as in your case you have only one B4ms VM, you can directly buy 1 quantity of B4ms VM reservation which will cover this VM 100%.

Again, to get the maximum discount, it is recommended that you purchase reservation for all the VMs which has consistent usage 24/7 and you purchase the reservation of the same VM size.

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Answer 2

Thank you @darsha-MSFT . If I were to have several VMs in the same family but with different sizes (lets say one B1ms, two B2ms and three B4ms), should I then purchase multiple reservations for the lowest size? What is best practice? Why should I purchase multiple reservations for the lowest size instead of for instance one reservations of B1ms, two for B2ms and three for B4ms?

Answer 3

Hello Charlotte

Please refer this document section: https://learn.microsoft.com/en-us/azure/virtual-machines/reserved-vm-instance-size-flexibility

This will give you more clarity on VM reservation purchase.

If you have several VMs of same family with different sizes, it's upto you to decide which would be best suited. Ideally purchasing reservation for VMs which are running continuously will be beneficial. In the example you can either choose multiple smaller size RI or one single bigger size RI. Either way, you have to ensure to enable instance flexibility so that the reserved hours don't go idle and gets utilized to any available VM at a point in time.

Regards,

Divya.