When scaling Azure based upon load, you can scale horizontally (add more compute instances) or vertically (increasing the size of the VM). Azure Auto-scaling functionality works with horizontal scaling.  For more details about these options see my previous blog post on “Cloud Scaling Patterns“.

Azure Scale Units (SU) apply to vertical scaling, where you are wanting to scale ‘up’ the unit on which your VM is partitioned.  As new hardware becomes available Microsoft builds them into a new “scale unit”. If scaling UP, there are some sizes you can’t scale up (resize) to depending upon what SU your VM is currently partitioned.  VMs can only be resized to a size supported in the SU where the VM is deployed. Currently there are five scale unit types:

  • Scale Unit 1: A0-A4 (original VM sizes) Basic VMs, no LB, no Autoscale, can only scale between A0-A4)
  • Scale Unit 2: A0-A7 (like SU1 but adds A5-A7)
  • Scale Unit 3: A8/A9 (“HPC” VMs, optimized networking with Infiniband)
  • Scale Unit 4: A0-A7 and D1-D14 (D’s series with SSDs and better CPUs, and all A0-A7)
  • Scale Unit 5: G1-G5 (monster powerful VMs up to 32 core Xeon CPUs/448 GB RAM/6596 GB SSD storage/64 data disks)

How to check the SU unit a VM is currently allocated

  1. VM/Configure tab
  2. Click on VM Size and drop down all sizes. Here, you see A0-A4 so you can tell what SU (#1) you are using. So you cannot scale up to anything above S4 in this case.

How to choose scale unit

  • To ensure you get a SU that will meet your needs for scaling up, ensure the first VM deployed in that Cloud Service (or legacy Affinity Group) is in the upper range. So if you want SU2, deploy an S5 or above (S6 or S7) and you will be in SU2 at that point for all subsequent allocations
  • When you deploy a smaller size, like A2, you can get put into many different scale units. You won’t necessarily be in SU1.

How to move to a bigger size VM that is not in your current SU:

  • If you are in SU1 and need to move to a VM size that is not in SU1 (say A5-A7 in SU2) you can’t change it directly from the UI
  1. Note the OS disk name in the usage overview
  2. Delete the disk but “Keep the Attached Disks”
  3. Go to VM/Disks and make sure that disk is not attached to any other VMs
  4. Go to Gallery and choose that OS Disk and select the upgraded size you want to upgrade it to.