VMware Validated Design for Software-Defined Data Center uses a set of building blocks called workload domains. A workload domain consists of a set of servers that are managed by one vCenter Server instance and network equipment for connection to the data center.
Workload Domain Architecture Characteristics
Workload domains can include different combinations of servers, and network equipment which can be set up with varying levels of hardware redundancy and varying quality of components. Workload domains are connected to a network core that distributes data between them.
A workload domain represents a logical boundary of functionality, managed by a single vCenter Server instance. The workload domain is not defined by any hard physical properties. Although a workload domain usually spans one rack, you can aggregate multiple workload domains in a single rack in smaller setups. For both small and large setups, consider homogeneity and easy replication .
Different workload domains of the same type can provide different characteristics for varying requirements. For example, one virtual infrastructure workload domain can use full hardware redundancy for each component (from power supply to memory chips) for increased availability. At the same time, another virtual infrastructure workload domain in the same setup could use low-cost hardware without any hardware redundancy. These variations make the architecture suitable for different workload requirements in the SDDC.
Workload Domain to Rack Mapping
Workload domains are not mapped one-to-one to data center racks. While a workload domain is an atomic unit of repeatable building blocks, a rack is a unit of size. Because workload domains can have different sizes, you map workload domains to data center racks according to the use case.
In a Layer 3 network fabric, the consolidated cluster cannot span racks. NSX Controller instances and other virtual machines rely on VLAN-backed networks. The physical network configuration ends Layer 2 networks in each rack at the Top of the Rack (ToR) switch. Therefore, you cannot migrate a virtual machine to a different rack because the IP subnet is available only in the rack where the virtual machine currently resides.
One Workload Domain in One Rack
One workload domain can occupy exactly one rack.
Multiple Workload Domains in One Rack
Two or more workload domains can occupy a single rack, for example, one management workload domain and one virtual infrastructure workload domain can be deployed to a single rack.
Single Workload Domain Across Multiple Racks
A single workload domain can stretch across multiple adjacent racks. For example, a virtual infrastructure workload domain that has more ESXi hosts than a single rack can support.