Re: What Is An Oracle Instance, Really ?

Jannette wrote:
>
> Pls educate me...I've read MANY explanations. But none of them seems to
> be in the context of reality.
> So if I have 4 running databases (4 service names) on one server with a
> single Oracle Home and a single
> Oracle80.exe, does that mean I have 4 Instances running ? ie is each
> database mapped to a one and one only Instance ?

Each "instance" is a set of UNIX background processes (at least including "pmon", "smon", "dbwr" and "lgwr") and a set of shared memory segments referred to as the "SGA". Unless you are running Oracle Parallel Server, each instance will manage a single database (a set of files) & each database will be managed by only one instance. OPS lets you have multiple instances managing a single database -- but the reverse is not possible: you cannot have a single instance m anage multipole databases. To simplify things, just think of running processes & memory-resident structures as making up the instance & the disk-resident files as making up the database. When you shut down Oracle or the machine, the instance goes away -- bu the database still exists on the filesystems. Finally the instance & the database usually have the same name but they don't have to. The instance name (the "sid") is basically a pointer to a set of files (like the init<sid>.ora) that configures the instance while the database name is stored internally in the Data Dictionary & in the database file headers.

Also, all Oracle processes aasociated with a single instance will execute the same shared-memory resident image of oracle.exe. And, AFAIK, on most UNIX ports all instances running out of the same ORACLE_HOME will also share the same executable image.

Hope that helps.
--

Vincent Ventrone    |  The MITRE Corp.
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