Oracle server architecture
One of the first things to understand about Oracle RDBMS (Relational Database Management System) is the difference between the terms: “database” and “instance”. This article will discuss the term “instance.” Briefly, the term instance refers to processes running in memory that disappear when the database is closed. This is different from database files that remain in the operating system’s file systems. , whether or not the database is running.
oracle instance
The Oracle instance is made up of shared memory: SGA (system global area)and a set of operating system processes and memory structures that manage database files.
The SGA is made up of 3 main areas in memory:
- Shared poolwhich contains various structures for executing SQL and tracking dependencies
- buffer cachecontaining images of data blocks read from the database
- log bufferwhich is used to store redo information before it is flushed to disk
Background processes of the instance:
The following are the main ones (there may be many more) UNIX processes that start and continue to run whenever the database is active:
- PMONthe process monitor periodically checks for processes that need cleaning
- SMONthe system monitor, merges free space within the database and verifies that all required instance processes are active
- DBWRthe database writer (DBWn if there are multiple processes) writes data blocks back to disk from the buffer cache
- LGWRthe log writer (LGWn if there are multiple processes) writes redo information to disk from the log buffer
- CKPTupdate file headers during a checkpoint
- BOWis an optional process that copies redo information to an archive area so that changes can be reapplied during recovery, if the database is in archive log mode
a common UNIX executing the command to see the oracle background processes (i.e. if the database instance is running) is:
ps-ef | pray grep
this command production Below are the main processes and some other typical processes seen in a single instance running SAP Oracle database. In this case, the name of the SID (System Identifier) database is “CLQ”. SAP database names, or SIDs, tend to be 3 characters long. The central section is the abbreviation for the processes:
ora_mman_CLQ
ora_arc0_CLQ
ora_arc1_CLQ
ora_gen0_CLQ
ora_dbrm_CLQ
ora_vktm_CLQ
hour_day0_CLQ
ora_diag_CLQ
ora_smco_CLQ
ora_pmon_CLQ
ora_psp0_CLQ
ora_dbw0_CLQ
ora_dbw1_CLQ
ora_lgwr_CLQ
ora_ckpt_CLQ
ora_smon_CLQ
ora_reco_CLQ
ora_mmon_CLQ
ora_mmnl_CLQ
ora_arc2_CLQ
ora_arc3_CLQ
ora_qmnc_CLQ
ora_q002_CLQ
ora_q000_CLQ
ora_w000_CLQ
ora_w001_CLQ
ora_vkrm_CLQ
ora_cjq0_CLQ
Congratulations!You have now learned one part of Oracle Server Architecture: what the term “instance.” This brief treatment is not intended to cover the topic in detail, but rather is intended as an introduction and a nice, easy way to master a complex topic. Next, you should learn about Oracle Database parameters records