Consideration vSphere 5 Auto Deploy: Stateless ESXi
The boot process proceeds as follows.
1 The administrator reboots the host.
2 As the host boots up, Auto Deploy provisions the host with its image and host profile, which are stored
in vCenter Server.
3 Virtual machines are brought up or migrated to the host based on the settings of the host.
n Standalone host. Virtual machines are powered on according to autostart rules defined on the host.
n DRS cluster host. Virtual machines that were successfully migrated to other hosts stay there. Virtual
machines for which no host had enough resources are registered to the rebooted host.
If the vCenter Server system is unavailable, the host contacts the Auto Deploy server for image profiles and
host profiles and the host reboots. However, Auto Deploy cannot set up vSphere distributed switches if vCenter Server is unavailable, and virtual machines are assigned to hosts only if they participate in an HA cluster. Until the host is reconnected to vCenter Server and the host profile is applied, the switch cannot be created and, because the host is in maintenance mode, virtual machines cannot start. See “Reprovision Hosts with Simple Reboot Operations,” on page 77.
The word I would like to bring out is maintenance mode. Without a vCenter any reboot of the host will result in one less host participated in the cluster.
I see this as a consideration to take note when using Auto Deploy. Without vCenter, the vDS and HA will be not configured on the host been the fact that the database with the host profile provided by vCenter is not present. (HA agent, FDM will not start as every reboot is equal to a new server joining to the cluster and in such HA will start to configure on the host.)
If this was on a stateful host, HA and vDS will not be lost as the ESXi server is able to start up the FDM agent and vDS setting is present.
Whether to use Auto Deploy or not is a big consideration and not as simple as it seems. I once thought its was a good tool not ware of the above. Though stateless ESXi has it advantages like easy update of ESXi state instead of updating all the servers one at a time or even security measure where data are sensitive and do not want to be present on the harddisk.
It has show that vCenter has taken a very important role in every single vSphere upgrade. If you are to use Auto Deploy ESXi servers, do note of the importance of vCenter and having vCenter with redundancy like vCenter Heartbeat and a regular backup that is valid is important.
I mention "valid" backup, as often I realize organization never perform any check on whether their backup are valid and only discover during Disaster Recovery were they to know their backup cannot be used.