part 2 in the “Tipd and Tricks for OVM” series. Part 1 is here
Every OVM Server receives an unique ID when the agent starts. Without this ID, a server can’t join a cluster and the manager can’t dispatch commands to the agent. But how does this ID get set?
Well if you look into the OVM source RPM’s for the ovs-agent-3.1.1-89.src, you’ll see in the file linux_xen/Extensions/discover.c file the following :
/* agent_unique_id – report a unique 16 byte identifier for this node
* Generates a 16 byte response string containing a unique 16 byte
* identifier for this node. Uses the SMBIOS UUID if it appears to be valid.
* Otherwise generate a fallback value based on ethernet MAC addresses.
* Returns non-zero status if no id can be constructed.
And a bit further
/* generate_fallback_unique_id – create a unique ID without using SMBIOS UUID
// Try using a sequence of ethernet MAC addresses as the unique ID.
// MAC’s are not good because the NIC could be replaced, or worse
// moved between systems. Using multiple MACs will catch a few
// more cases (unless it’s a multiport NIC, sigh).
So We can conclude that the agent will :
- Ask SMBIOS for an id and if it’s valid use it. If not valid or no ID then
- Get the MAC address of physical eth devices on the system and concatenate in an UUID
This poses a great risk because if for some reason you change the motherboard ( other SMBIOS ) or 1 of the network devices on the system ( other MAC ), you’ll have a new UUID.
How to detect a changed UUID?
The following points to a changed uuid :
- Unable to join cluster after reboot
- “Unable to send notification” messages on the console
- The OVM Manager fails to rediscover the server after reboot ( tries 5 times and gives up )
- The OVM Server in OVM Manager will become in ERROR state with an error that looks like : “The server has changed IP or is unreachable”
Fakeuuid it !
In order to solve this issue, the agent has a ini parameter you can set to ensure the same UUID all the time. You can find it in the file /etc/ovs-agent/agent.ini. Get the server uuid from OVM Manager and set it in this parameter.
I personally set this on every OVM install i’m doing to ensure there are no issues with this ID changing.