When you install Oracle, on a very high abstraction level, you copy files to your HD. Nothing more nothing less.
After this process, you got an Oracle installation: a system, with at least one directory where Oracle’s programs reside, but you don’t have any Instance.
An Oracle Instance is the software (and memory) that Oracle uses to manipulate the data in the database. An Instance can by identified by its Oracle SID (System IDentifier) and is made up of Oracle system files (like the Init file which is where all Oracle’s parameters resides, and files where Oracle store its own structures and internal data) and of user files (files that store your data). An Instance (when running) consume RAM and CPU.
Every Oracle’s Instance has got:
- A SID. SIDs must be unique inside a Server
- An Init<SID>.ora. The Init file with all Oracle’s parameters inside, its complete name is like that (you must substitute your SID to <SID>, so if you got a SID equal to MYORA the Init would be InitMYORA.sid)
- Some memory assigned to
- A bunch of system data file like the system.ora data file (it’s like a user data file but it stores only Oracle’s own data)
- A bunch of user data file
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