Oracle database startup process

[email protected] zxy>startup
ORACLE instance started.

Total System Global Area  830930944 bytes
Fixed Size                  2232920 bytes
Variable Size             520097192 bytes
Database Buffers          306184192 bytes
Redo Buffers                2416640 bytes
Database mounted.
Database opened.

look at the whole process of database startup, the first is to start the database instance, the instance starts, load SGA, allocate shared memory, start the background process, then load various library files, and then start the database. In fact, there are three states in total. You can also see the instance startup, mount, open, and the following summary:

1 NOMOUNT: Start the database instance, read the parameter file at this time, but do not load the database (this mode only Create an instance, create various memory structures and service processes of the Oracle instance, 5 of which must be started, DBWR, LGWR, SMON, PMON, CKPT, do not load the database, will load the parameter file, and will not open any data File);

2 MOUNT: Start the database instance, load the database, open the control file, but the database is closed;

3 OPEN: Start the database instance, load and open the database, this time the data file is opened, The data can also be accessed normally;