Many
people use the terms
instance and database
interchangeably, but the instance and the database are actually
separate entities. Let’s look at what happens when the database
is started from the Server Manager utility
(svrmgrl
, svrmgrm
, or
svrmgr30
for Oracle8 on Windows NT). The startup
process follows:
The background processes are started.
The System Global Area (SGA) is allocated in memory.
The background processes open the various files.
The database is ready for use.
During the startup, messages are displayed that notify you of what is happening. Here is a sample startup sequence:
SVRMGR> startup ORACLE instance started. Total System Global Area 11865072 bytes Fixed Size 33708 bytes Variable Size 10672196 bytes Database Buffers 1126400 bytes Redo Buffers 32768 bytes Database mounted. Database opened.
You can see that the instance is started before the files that constitute the database are opened. The instance consists of the background processes and the SGA. The SGA totals are listed before the database is started but after the “ORACLE instance started” message. The database is the collection of logical objects and physical files necessary to support the system, and the database system is the instance, SGA, and files. Figure 2.1 shows the components of the database system after the database has been started.
Now, while what we’ve said is technically accurate, you’ll find the actual usage at Oracle sites to be quite different. Many people use the terms “instance,” “database,” and “database system” interchangeably. In this context, they are referring to the entire system: the background processes, the System Global Area, and the data, control, and redo log files.
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