The vSphere Auto Deploy network boot mechanism enables quick provisioning and configuration of ESXi hosts. Essentially, instead of managing the state of each host separately, with vSphere Auto Deploy you put in place centralized configurations and rules that map configurations to sets of ESXi hosts, ensuring that every time a host boots, its gets the same required state. When you boot ESXi hosts using vSphere Auto Deploy, the host server loads the ESXi image directly into memory and therefore does not have to store the ESXi state in any type of persistent storage. Managing hosts in this type of centralized manner allows you to easily scale out the management of significantly more physical servers and have a more deterministic environment, simplifying day-to-day operational tasks.

When you start a physical host that is configured to use it, vSphere Auto Deploy employs a PXE boot infrastructure in conjunction with vSphere host profiles to provision and customize that host. No state information is typically stored on the hypervisor itself. Instead, the vSphere Auto Deploy server manages the state information for each host individually.

Introduced in vSphere 5.1, the stateless caching feature allows the host to cache its state information by forcing the hypervisor to store the ESXi image and configuration on a local disk, a remote disk, or a USB drive. Subsequent boots, when the stateless caching feature is enabled, continue to provision the host through the vSphere Auto Deploy server. However, if the vSphere Auto Deploy server is not available, the host boots from cached image.

In addition to stateless caching, vSphere 5.1 introduced the ability to use vSphere Auto Deploy to perform stateful installations. In some designs, a vSphere Auto Deploy infrastructure can be employed to provision hosts that will perform all subsequent boots from its local persistent storage. This mechanism allows vSphere Auto Deploy to provision an ESXi host and configure a host profile that forces the host to store the ESXi image and configuration on the local disk, a remote disk, or a USB drive in the way described previously. In this type of use case, all subsequent boots of the ESXi host take place from the local image with no further interaction with the vSphere Auto Deploy infrastructure. This process is similar to performing a scripted installation and not dissimilar to a kickstart scripted installation, where the script provisions the host and the host then boots from disk. However, in this case, vSphere Auto Deploy provisions the host and the host then boots from its own media. This feature has meant, to some extent, that other community-driven deployment appliances, such as the ESX Deployment Appliance (EDA) and the Ultimate Deployment Appliance (UDA), have lost momentum.