An administrator is upgrading an ESXi 5.5 host to ESXi 6.x and gets the following error: MEMORY_SIZE What does this indicate?

ESXi 6.x requires a minimum of 4 GB of physical RAM on the host, and the pre-upgrade compatibility check raises MEMORY_SIZE when this threshold is not met. This check runs against the target ESXi host hardware directly, preventing the upgrade from proceeding on under-resourced hardware. Resolving the error requires adding physical memory to the host before retrying the upgrade.

Auto Deploy is a stateless provisioning mechanism that PXE-boots hosts; it does not perform or generate MEMORY_SIZE errors related to a host's physical RAM during an upgrade.

vCenter Server has its own separate memory requirements, and a vCenter resource constraint would surface as a different error unrelated to the ESXi host hardware check.

Update Manager orchestrates the upgrade workflow but the MEMORY_SIZE check reflects the target ESXi host's physical memory, not Update Manager's own resource availability.