Q: Why do I sometimes seem to hit limitations when adding new virtual machines to an existing cloud service or resizing a virtual machine?
A: Azure deploys servers in scale units, also known as stamps and clusters. Typically these scale units consist of about 20 racks, with nearly 1,000 servers total. All servers in a scale unit use the same hardware. As Microsoft introduces new series and sizes of virtual machines, new hardware might be required to support the new types of virtual machine; this hardware is rolled out in scale units. A cloud service resides in a single scale unit, which means that if your existing cloud service is running in an older scale unit it won't be able to add newer series/size virtual machines to it. Currently, the following virtual machines are supported by the various scale units:
- Type 1: A0-A4
- Type 2: A0-A7
- Type 3: A8/A9
- Type 4: A0-A7 and D1-D14
- Type 5: G1-G5 (Godzilla)
- Type 6: DS1-DS14
This means your cloud service is bound to one of these types, depending on the first virtual machine created. Therefore, if your cloud service is currently running on a type 1 scale unit, then you can only add A0-A4 virtual machines; if it's running on a type 4 scale unit, you have the broadest range of virtual machines supported.
To give yourself the most flexibility when creating a new cloud service, make the first virtual machine deployed a D series virtual machine. This means you'll be deployed to a type 4 scale unit and therefore have the widest range of virtual machine sizes available. The following figure shows all the sizes available that I can reconfigure my virtual machine to. Because I have A0-A7 and D1-D14, it means I am on a type 4 scale unit.
Remember that using a virtual network means virtual machines in different cloud services can communicate directly with each other; therefore, having multiple cloud services isn't a problem and doesn't hamper communication in any way between your workloads.
Note that virtual machines can only be resized to a size supported on the scale unit where the virtual machine is deployed. This means I couldn't resize an A4 virtual machine to an A8 and possibly not a D series, depending on the scale unit deployed to. If you need to perform a resize outside of the sizes available in the configuration option, you should delete the virtual machine but select the option to keep the attached storage and then create a new virtual machine in a new cloud service (or a cloud service that supports the new size) and reattach the disks from the old virtual machine.