Oracle RAC GRID Daemons and Background process

Oracle Cluster Specific Daemons:

Crsd :

The CRS daemon (crsd) manages cluster resources based on configuration information that is stored in Oracle Cluster Registry (OCR) for each resource. This includes start, stop, monitor, and failover operations. The crsd process generates events when the status of a resource changes.

Cssd :

 Cluster Synchronization Service (CSS): Manages the cluster configuration by controlling which nodes are members of the cluster and by notifying members when a node joins or leaves the cluster. If you are using certified third-party clusterware, then CSS processes interfaces with your clusterware to manage node membership information. CSS has three separate processes: the CSS daemon (ocssd), the CSS Agent (cssdagent), and the CSS Monitor (cssdmonitor). The cssdagent process monitors the cluster and provides input/output fencing. This service formerly was provided by Oracle Process Monitor daemon (oprocd), also known as OraFenceService on Windows. A cssdagent failure results in Oracle Clusterware restarting the node. 

Diskmon :

Disk Monitor daemon (diskmon): Monitors and performs input/output fencing for Oracle Exadata Storage Server. As Exadata storage can be added to any Oracle RAC node at any point in time, the diskmon daemon is always started when ocssd is started. 

Evmd :

Event Manager (EVM): Is a background process that publishes Oracle Clusterware events 

Mdnsd :

Multicast domain name service (mDNS): Allows DNS requests. The mDNS process is a background process on Linux and UNIX, and a service on Windows. 

Gnsd :

Oracle Grid Naming Service (GNS): Is a gateway between the cluster mDNS and external DNS servers. The GNS process performs name resolution within the cluster. 

Ons :

Oracle Notification Service (ONS): Is a publish-and-subscribe service for communicating Fast Application Notification (FAN) events 

Oraagent :

oraagent: Extends clusterware to support Oracle-specific requirements and complex resources. It runs server callout scripts when FAN events occur. This process was known as RACG in Oracle Clusterware 11g Release 1 (11.1). 

Orarootagent :

Oracle root agent (orarootagent): Is a specialized oraagent process that helps CRSD manage resources owned by root, such as the network, and the Grid virtual IP address 

Oclskd :

Cluster kill daemon (oclskd): Handles instance/node evictions requests that have been escalated to CSS .

Gipcd :

Grid IPC daemon (gipcd): Is a helper daemon for the communications infrastructure 

Ctssd :

Cluster time synchronisation daemon(ctssd) to manage the time syncrhonization between nodes, rather depending on NTP. 

 RAC Background Process:

LMSn — Global Cache Service Process: It can mainly handle the cache fusion part. It handles the consistent copies of blocks that are transferred between instances. It receives the request from the LMD to perform lock requests. It rolls back any uncommitted transactions. There can be up to 10 LMS process running and can be started dynamically if demand requires. It also handles the global deadlock detections and monitors for the lock conversion timeouts.

LMON    — Global Enqueue Service Monitor: This process manages the GES, it maintains the consistency of GCSmemory in case of process death. It also responsible for cluster reconfiguration and locks reconfiguration.

LMD     — Global Enqueue Service Daemon: This manages eneque manager service request for the GCS. It also handles the deadlock detections and remote resource requests the other instances.

LCK0    — Instance Enqueue Process: Manages the instance resource requests and cross instance call operations for shared resources. It builds a list of invalid lock elements and validates the lock elements during recovery.

DIAG    — Diagnosability Daemon

GCS ensures a single system image of the data even though the data is accessed by multiple instances.

GES maintains or handles the synchronization of the dictionary cache, library cache, transaction locks, and DDL locks. In other words, GES manages enqueues other than data blocks. To synchronize access to the data dictionary cache, latches are used in exclusive (X) mode and in single-node cluster databases. Global enqueues are used in cluster database mode