Proximity placement group vs availability zones vs availability set
Verdict: Use a proximity placement group only when default placement misses the app-to-DBMS latency target. Spread across availability zones for resiliency; one group cannot span zones. Combine an availability set with a PPG to pin it to a zone.
| Criterion | Proximity placement group | Availability zones | Availability set | Default placement |
|---|---|---|---|---|
| Optimises for | Lowest app-to-DBMS latency | Zone-level fault isolation | Fault/update-domain split inside one zone | The norm; sufficient in most cases |
| Zone span | Cannot span zones | Spans separate zones by design | Pinned into one zone via a PPG | Region default |
| Main risk | AllocationFailure on elastic add/remove and mixed VM families | No guaranteed inter-zone distance or latency | Only platformFaultDomainCount=1 supported for SAP | May miss a strict latency target |
| Use when | Measured latency exceeds the requirement | Datacentre-failure resiliency is the priority | ASCS/ERS needs FD/UD split in one zone | No measured latency problem |
Rules
- Proximity placement groups are not the default; use them only when default placement fails the roundtrip latency requirement between SAP application servers and the DBMS.
- For lowest latency use a PPG within one availability zone; for highest resiliency spread instances across zones, since one PPG cannot span zones.
- To combine an availability set with a specific zone, pin the set to that zone through a PPG; only platformFaultDomainCount=1 is supported for SAP.
- Even within one zone latency can affect SAP when the zone spans multiple datacentres, which is part of the motivation for a PPG.
- Some VM families cannot share a PPG and cross-family resizing is restricted, so limit PPG use to systems that need it.
Traps
- Placing every SAP VM in a PPG by default causes AllocationFailure on elastic add/remove and mixed-family deployments.
- Availability zones do not guarantee a fixed distance or identical latency between zones across regions.
- Standard Load Balancer, not Basic, is mandatory for zone-aware SAP cluster IPs.