Question

What does it mean, if the Oracle listener status (util O / 2 / 4) shows status 'UNKNOWN'?

Service "aleph22.alephserver.xxx.com" has 2 instance(s). 
Instance "aleph22", status UNKNOWN, has 1 handler(s) for this service... 
Instance "aleph22", status READY, has 1 handler(s) for this service... 

Answer

Oracle's explanation for the "UNKNOWN" status handler is this - 

"An UNKNOWN status means that the instance is registered statically (with a SID_LIST) in the listener.ora file rather than dynamically with service registration. Therefore, the database status is "not known" to the listener. There is no reason a connection cannot be made, however, using the SID value. 

For normal communication, the SID_LIST in the listener.ora file is not required. One of the reasons why Oracle recommends not using one, is because it may cause un-necessary overheads on the listener process. The SID_LIST section only needs to be used for instances such as PLSExtProc or if older OEM tools are in use. It is not normally a problem having the instance duplicated in the SID_LIST, but it is preferable not to have it." 

We have the SID_LIST configured to specify ORACLE_HOME in listener.ora, like this:

sid_list_listener=(sid_list= 
(sid_desc= 
(global_dbname=aleph22.alephserver.xxx.com) 
(sid_name=aleph22) 
(oracle_home=/exlibris/app/oracle/product/11r2) 
) 
) 

Thus the overhead. But the application should have no problem to connect to the database using SID