Oracle Architecture Understanding (1)

Oracle is designed to be a very portable database software that can be run more stably, whether in Linux,unix or Windows download, and is supported on mainframes.

This blog is only used to document the initial understanding of Oracle architecture when a person first learns oracle, and if there is a shortage, I hope you will correct it.

Databases and instances

Database: A collection of physical operating system files or disks. When using ASM (Automatic storage Management) for Oracle, or when using raw partitions, the database may not be a separate file in the file system, but the definition remains the same.

Note: ASM was temporarily interpreted as a management mechanism with similar functions to LVM for the first time when I approached it.

Instance (instance): A set of Oracle background processes (under Linux and Unix) and a shared memory area (memory structure), even if there is no disk storage, the instance can exist.

Note: The database can be loaded and opened by multiple instances, and instances can be reproduced and opened in just one database at any point in time. That is, at the same time, only one database can be accessed and interacted with by the current instance.

When we perform a software-only database operation and do not include the behavior of the "start" database, there is nothing except the software.

At this point the $oracle_home/dbs directory (ORACLE's working directory) does not have any files, we use the Ls-l view will find information:

Total 0

There are no Init.ora files (parameter file parameter files), and the PS command will not find any process on Oracle (in addition to the Oracle keyword that occurs with grep Oracle operations), if the build database executes DBCA The Init.ora parameter file is generated when the Sqlplus/as SYSDBA is executed, and the instance name specified by $oracle_sid is not authenticated as SYSDBA with the user's password.

after accessing the database, the PS aux on the system will find many processes, they start with Ora_, including Smon (System monitoring), CKPT (checkpoint), Pmon (process monitoring) and other processes.

Note: an instance can only load and open an instance in its lifetime, if the database is closed (alter close), then the lifetime is complete (reclaim memory space, end the related process), and if you want to open a database again, you need to discard the instance. Create a new instance (try shutdown to close the instance and then start again to turn on the equivalent of re-establishing the instance to open the database).

This is the division of Oracle's instance and database concepts.

Oracle Architecture Understanding (1)