The VMware Validated Design for SDDC uses a small set of common building blocks called pods.
Pod Architecture Characteristics
Pods can include different combinations of servers, storage equipment, and network equipment, and can be set up with varying levels of hardware redundancy and varying quality of components. Pods are connected to a network core that distributes data between them. The pod is not defined by any hard physical properties, as it is a standard unit of connected elements within the SDDC network fabric.
A pod is a logical boundary of functionality for the SDDC platform. While each pod usually spans one rack, it is possible to aggregate multiple pods into a single rack in smaller setups. For both small and large setups, homogeneity and easy replication are important.
Different pods of the same type can provide different characteristics for varying requirements. For example, one compute pod could use full hardware redundancy for each component (power supply through memory chips) for increased availability. At the same time, another compute pod in the same setup could use low-cost hardware without any hardware redundancy. With these variations, the architecture can cater to the different workload requirements in the SDDC.
One of the guiding principles for such deployments is that VLANs are not spanned beyond a single pod by the network virtualization layer. Although this VLAN restriction appears to be a simple requirement, it has widespread impact on how a physical switching infrastructure can be built and on how it scales.
Pod to Rack Mapping
Pods are not mapped one-to-one to 19" data center racks. While a pod is an atomic unit of a repeatable building block, a rack is merely a unit of size. Because pods can have different sizes, how pods are mapped to 19" data center racks depends on the use case.
One Pod in One Rack. One pod can occupy exactly one rack.
Multiple Pods in One Rack. Two or more pods can occupy a single rack, for example, one management pod and one shared edge and compute pod can be deployed to a single rack.
Single Pod Across Multiple Racks. A single pod can stretch across multiple adjacent racks. For example, a storage pod with filer heads and disk shelves can span more than one rack or a compute pod that has more host then a single rack can support.
The mangement and the shared edge and compute pods can not span racks. This is due to NSX controllers and other virtual machines on a VLAN backed network migrating to a different rack where that IP subnet is not available due to layer 2 termination at the Top of Rack switch.