With VMs hosted in Azure you need a fine balance between protection from hardware failure on the underlying Azure platform, plus performance from having the tiers of your SAP application being physically close together.
For this very purpose, Microsoft introduced Proximity Placement Groups (PPGs) to allow an administrator to ensure that specific tiers (e.g. application and database) are located close. Potentially even in the same server rack.
The PPGs also affect the location of the storage assigned to the VMs, although the storage infrastructure is actually transparent to administrators.
The PPGs still allow Azure to honor the Availability Sets, Fault Domains and Update Domains.
In this post, I show a method of finding the physical hostname of your Windows VM which could be part of a check before/after implementing a PPG.
NOTE: PPGs should be created at the time a VM is created, and assigned to the “lead” system of the rarest size. Example, an M-series VM is rare, so this should be the lead system when creating the PPG. This will anchor the other VMs to this M-series VMs location.
The previous blog post shows how to do this for a Linux VM.
On a Windows VM in Azure, as any Windows user with access to the registry, you can use the following to see the name of the physical host on which your VM is running:
reg query "HKEY_LOCAL_MACHINE/Software/Microsoft/Virtual Machine/Guest/Parameters" /v PhysicalHostName
PhysicalHostName REG_SZ DUB012345678910
In this case, we take the first 3 chars to be “Dublin”, which is in the EU North Azure region.
The remaining characters consist of the rack and physical hostname.
If you have 2 VMs in the same rack on the same physical host, then you will have minimal latency for networking between them.
Conversely, if you have 2 VMs on the same physical host, you are open to HA issues.
Therefore, for SAP, you need a good balance of distance.
You should expect to see SAP S/4HANA application servers and HANA DBs placed in the same Proximity Placement Groups, within the same rack, even potentially on the same host (providing you have availability sets across the tiers you will be safe).