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DBA - Process: Background Processes

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• DBA - Process: Background Processes
  • Background Processes
  • Mandatory Background Processes
    • Process Monitor Process (PMON) Group
      • Database Resource Quarantine
    • Process Manager (PMAN)
    • Listener Registration Process (LREG)
    • System Monitor Process (SMON)
    • Database Writer Process (DBW)
    • Log Writer Process (LGWR)
      • LGWR and Commits
      • LGWR and Inaccessible Files
    • Checkpoint Process (CKPT)
    • Manageability Monitor Processes (MMON and MMNL)
    • Recoverer Process (RECO)
  • Optional Background Processes
    • Archiver Processes (ARCn)
    • Job Queue Processes (CJQ0 and Jnnn)
    • Flashback Data Archive Process (FBDA)
    • Space Management Coordinator Process (SMCO)

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Background Processes

• Background processes

  • created by Oracle Database automatically when a database instance starts.
  • additional processes used by a multiprocess Oracle database. - perform maintenance tasks required to operate the database and to maximize performance for multiple users.

• Each background process has a separate task, but works with the other processes. 任务独立, 但相互配合.

  • e.g., the LGWR process writes data from the redo log buffer to the online redo log. When a filled redo log file is ready to be archived, LGWR signals another process to archive the redo log file.

• An instance can have many background processes, not all of which always exist in every database configuration.

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• Command to query background processes

SELECT PNAME
FROM   V$PROCESS
WHERE  PNAME IS NOT NULL
ORDER BY PNAME;

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Mandatory Background Processes

• Mandatory background processes

  • presented in all typical database configurations.
    • run by default in a read/write database instance started with a minimally configured initialization parameter file.
    • A read-only database instance disables some of these processes.

• mandatory background processes:

  • Process Monitor Process (PMON) Group
  • Process Manager (PMAN)
  • Listener Registration Process (LREG)
  • System Monitor Process (SMON)
  • Database Writer Process (DBW)
  • Log Writer Process (LGWR)
  • Checkpoint Process (CKPT)
  • Manageability Monitor Processes (MMON and MMNL)
  • Recoverer Process (RECO)

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Process Monitor Process (PMON) Group

• The PMON group includes PMON, Cleanup Main Process (CLMN), and Cleanup Helper Processes (CLnn).

  • These processes are responsible for the monitoring and cleanup of other processes.

• The PMON group oversees cleanup of the buffer cache and the release of resources used by a client process.

  • e.g.: the PMON group is responsible for
    • resetting the status of the active transaction table,
    • releasing locks that are no longer required,
    • and removing the process ID of terminated processes from the list of active processes.

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• Process Monitor Process (PMON) 监控

  • detects the termination of other background processes. 监控
  • If a server or dispatcher process terminates abnormally, then the PMON group is responsible for performing process recovery.

• Cleanup Main Process (CLMN)清理

  • PMON delegates cleanup work to the cleanup main process (CLMN).
  • The task of detecting abnormal termination remains with PMON.
  • periodically performs cleanup of terminated processes, terminated sessions, transactions, network connections, idle sessions, detached transactions, and detached network connections that have exceeded their idle timeout.

• Cleanup Helper Processes (CLnn) 帮助清理

  • CLMN delegates cleanup work to the CLnn helper processes.
  • assist in the cleanup of terminated processes and sessions. The number of helper processes is proportional to the amount of cleanup work to be done and the current efficiency of cleanup.
    • Oracle Database can use multiple helper processes in parallel to perform cleanup, thus alleviating slow performance.可以多个同时

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• V$CLEANUP_PROCESS:

  • provides information on the PMON processes.

• V$DEAD_CLEANUP:

  • shows the dead processes and killed sessions present in the instance and their cleanup status.

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Database Resource Quarantine

• If a process or session terminates, then the PMON group releases the held resources to the database.

  • In some cases, the PMON group can automatically quarantine corrupted, unrecoverable resources so that the database instance is not immediately forced to terminate.自动隔离
  • The PMON group continues to perform as much cleanup as possible on the process or session that was holding the quarantined resource.尽可能清除

• V$QUARANTINE: provides information about quarantined objects.

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Process Manager (PMAN)

• Process Manager (PMAN)

  • oversees several background processes including shared servers, pooled servers, and job queue processes.

• PMAN monitors, spawns, and stops the following types of processes:

  • Dispatcher and shared server processes
  • Connection broker and pooled server processes for database resident connection pools
  • Job queue processes
  • Restartable background processes

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Listener Registration Process (LREG)

• listener registration process (LREG)

  • registers information about the database instance and dispatcher processes with the Oracle Net Listener.

• When an instance starts, LREG polls the listener to determine whether it is running.

  • If the listener is running, then LREG passes it relevant parameters.
  • If it is not running, then LREG periodically attempts to contact it.

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System Monitor Process (SMON)

• system monitor process (SMON)

  • in charge of a variety of system-level cleanup duties.

• Duties:

  • Performing instance recovery, if necessary, at instance startup.

    • In an Oracle RAC database, the SMON process of one database instance can perform instance recovery for a failed instance.

  • Recovering terminated transactions that were skipped during instance recovery because of file-read or tablespace offline errors.

    • SMON recovers the transactions when the tablespace or file is brought back online.

  • Cleaning up unused temporary segments.

    • e.g., Oracle Database allocates extents when creating an index. If the operation fails, then SMON cleans up the temporary space.

  • Coalescing contiguous free extents within dictionary-managed tablespaces.

• SMON checks regularly to see whether it is needed. 定期检查
• Other processes can call SMON if they detect a need for it. 可以被调用

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Database Writer Process (DBW)

• database writer process (DBW)

  • writes the contents of database buffers to data files.
  • write modified buffers in the database buffer cache to disk.

• one database writer process (DBW0) is adequate for most systems. 一个够用

  • can configure additional processes to improve write performance. 可以有多个
  • These additional DBW processes are not useful on uniprocessor systems.单进程系统无用

• Conditions to write dirty buffers to disk:

  • When a server process cannot find a clean reusable buffer after scanning a threshold number of buffers, it signals DBW to write. DBW writes dirty buffers to disk asynchronously if possible while performing other processing.
  • DBW periodically writes buffers to advance the checkpoint, which is the position in the redo thread from which instance recovery begins.
    • The log position of the checkpoint is determined by the oldest dirty buffer in the buffer cache.
• the writes tend to be slower than the sequential writes performed by LGWR. LGWR 是顺序写入, 快; DBW 是离散写入 block, 慢;
  • In many cases the blocks that DBW writes are scattered throughout the disk.
• DBW performs multiblock writes when possible to improve efficiency. 多从写入
  • The number of blocks written in a multiblock write varies by operating system.

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Log Writer Process (LGWR)

• log writer process (LGWR)

  • manages the online redo log buffer.
  • writes one portion of the buffer to the online redo log.

• Circumstances to write all redo entries

  • A user commits a transaction.
  • An online redo log switch occurs.
  • Three seconds have passed since LGWR last wrote.
  • The redo log buffer is one-third full or contains 1 MB of buffered data.
  • DBW must write modified buffers to disk.

• Before DBW can write a dirty buffer, the database must write to disk the redo records associated with changes to the buffer (the write-ahead protocol). Redo 先于 buffer

  • If DBW discovers that some redo records have not been written, it signals LGWR to write the records to disk, and waits for LGWR to complete before writing the data buffers to disk.如果 redo 未写入, 则等待.

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LGWR and Commits

• fast commit mechanism

  • used to improve performance for committed transactions.
• When a user issues a COMMIT statement, the transaction is assigned a system change number (SCN).
• LGWR puts a commit record in the redo log buffer and writes it to disk immediately, along with the commit SCN and transaction’s redo entries.

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LGWR and Inaccessible Files

• LGWR writes synchronously to the active mirrored group of online redo log files. 同时写多个
  • If a log file is inaccessible, then LGWR continues writing to other files in the group and writes an error to the LGWR trace file and the alert log.单个失败将继续, 会有警告
  • If all files in a group are damaged, or if the group is unavailable because it has not been archived, then LGWR cannot continue to function. 全部文件写入失败, 则不再继续.

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Checkpoint Process (CKPT)

• checkpoint process (CKPT)

  • updates the control file and data file headers with checkpoint information. 与 cf 和 df 头部有关
  • signals DBW to write blocks to disk. 反馈给 DBW

• Checkpoint information includes

  • the checkpoint position,
  • SCN,
  • and location in online redo log to begin recovery.当恢复时用于与 redo log 对齐.

• CKPT does not write data blocks to data files or redo blocks to online redo log files.

  • 与数据 block 与 redo block 无关

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Manageability Monitor Processes (MMON and MMNL)

• manageability monitor process (MMON)

  • performs many tasks related to the Automatic Workload Repository (AWR).
  • e.g.,MMON writes when a metric violates its threshold value, taking snapshots, and capturing statistics value for recently modified SQL objects.

• manageability monitor lite process (MMNL)

  • writes statistics from the Active Session History (ASH) buffer in the SGA to disk.
  • MMNL writes to disk when the ASH buffer is full.

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Recoverer Process (RECO)

• In a distributed database, the recoverer process (RECO) automatically resolves failures in distributed transactions.

• The RECO process of a node automatically connects to other databases involved in an in-doubt distributed transaction.

  • When RECO reestablishes a connection between the databases, it automatically resolves all in-doubt transactions, removing from each database’s pending transaction table any rows that correspond to the resolved transactions.

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Optional Background Processes

• optional background process

  • any background process not defined as mandatory.
  • Most optional background processes are specific to tasks or features.
    • e.g., background processes that support Oracle ASM are only available when this feature is enabled.

• some common optional processes:

  • Archiver Processes (ARCn)
  • Job Queue Processes (CJQ0 and Jnnn)
  • Flashback Data Archive Process (FBDA)
  • Space Management Coordinator Process (SMCO)

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Archiver Processes (ARCn)

• archiver process (ARCn)

  • copies online redo log files to offline storage after a redo log switch occurs.
  • also collect transaction redo data and transmit it to standby database destinations.

• ARCn processes exist only when the database is in ARCHIVELOG mode and automatic archiving is enabled.

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• Lab Change Archive Lod Mode

ARCHIVE LOG LIST        # check the current mode
SHUTDOWN IMMEDIATE
STARTUP MOUNT           # startup and mount the database
ALTER DATABASE ARCHIVELOG;  # alter db
ALTER DATABASE OPEN;        # Open the database
ARCHIVE LOG LIST            # check the current mode

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Job Queue Processes (CJQ0 and Jnnn)

• job

  • a user-defined task scheduled to run one or more times.

• queue process

  • runs user jobs, often in batch mode.

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Flashback Data Archive Process (FBDA)

• flashback data archive process (FBDA)

  • archives historical rows of tracked tables into Flashback Data Archives.

• When a transaction containing DML on a tracked table commits, this process stores the pre-image of the changed rows into the Flashback Data Archive. It also keeps metadata on the current rows.

• FBDA automatically manages the Flashback Data Archive for space, organization, and retention.
• Additionally, the process keeps track of how long the archiving of tracked transactions has occurred.

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Space Management Coordinator Process (SMCO)

• SMCO process

  • coordinates the execution of various space management related tasks.
• Typical tasks include proactive space allocation and space reclamation.
• SMCO dynamically spawns slave processes (Wnnn) to implement the task.

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