Question: Can Oracle support multiple
instances on different releases of Oracle on a single server?
Answer: Yes, Oracle is designed such that
dozens of instances can run on a server, each on a different release
of Oracle. Originally, the 1970's mainframes were designed to
support many databases on a single server, so this is not a new
concept, and multiple databases on a server is not unique to Oracle.
See here, there benefits of
multiple databases per server
and these important notes on
adding a new instance to an existing Oracle server.
The "one instance, one server" is a leftover from the 1990's days
of minicomputers, and today we are seeing massive
tips with many instances residing on giant million dollar
servers with hundreds of gigabytes of RAM.
In 2015 Oracle noted that 99% of OLTP applications have multiple
instances per server
Each instance can have it's own unique ORACLE_HOME directory,
each with a different release of Oracle, and the oratab file (/etc/oratab,
or /var/opt/oratab on Solaris, associates each instance
with a distinct Oracle home.
If you have a PC running Windows, there would be separate
registry entries for each instances, and the unique registry entries
can point to different ORACLE_HOME directories, each with multiple
releases of Oracle.
To see how many database cohabit a server, see this script to
instances on an Oracle server.
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