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 when:

• 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