Database Background Processes


Background Processes Maintains and enforces relationships between physical and memory structures
There are two types of database processes:

1.      Mandatory background processes
2.      Optional background processes

Mandatory background processes:
– DBWn, PMON, CKPT,  LGWR,  SMON

Optional background processes:
– ARCn, LMDn, RECO, CJQ0, LMON, Snnn, Dnnn, Pnnn, LCKn, QMNn

DBWn writes when:
• Checkpoint occurs
• Dirty buffers reach threshold
• There are no free buffers
• Timeout occurs
• RAC ping request is made
• Tablespace OFFLINE
• Tablespace READ ONLY
• Table DROP or TRUNCATE
• Tablespace BEGIN BACKUP

Log Writer (LGWR) writes:
• At commit
• When one-third full
• When there is 1 MB of redo
• Every three seconds
• Before DBWn writes

System Monitor (SMON) Responsibilities:
• Instance recovery
– Rolls forward changes in redo logs
– Opens database for user access
– Rolls back uncommitted transactions
• Coalesces free space
• Reallocates temporary segments

Process Monitor (PMON) Cleans up after failed processes by:
• Rolling back the transaction
• Releasing locks
• Releasing other resources
• Restarting dead dispatchers

Checkpoint (CKPT) Responsible for:
• Signaling DBWn at checkpoints
• Updating datafile headers with checkpoint information
• Updating control files with checkpoint information

Archiver (ARCn)
• Optional background process
• Automatically archives online redo logs when ARCHIVELOG mode is set
• Preserves the record of all changes made to the database

No comments:

Post a Comment