size=+0>
2.2 The Instance and the Database: Starting an Oracle Database
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.
Figure 2.1. Components of the database system after
startup
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.
What the Oracle System Files Really Are
The Oracle components we discuss in this chapter are all actually
just operating system files (except for the SGA, which is a
memory-resident structure). The physical database files must be
protected at the operating system level from intrusive access by any
user. These files are never written to directly by any user
application, though they are written to on behalf of user processes.
Oracle owns these files and will manage them. Users should never
have any operating system privileges on these files beyond the
privileges Oracle instructs you to set in the installation guide for
your system.
You name the physical database files when the database is
initialized and when additional datafiles are added. There are no
mandatory names for any of these files. You, as the DBA, can select
their names. The only restrictions are that the names cannot contain
blanks or special characters, and they must conform to the operating
system name requirements, and cannot exceed 30 characters in
length. |
|
No comments:
Post a Comment