Background Processes in a RAC database

LMS

This background process copy read consistent blocks from the holding instance buffer cache to the requesting instance. LMSn also performs rollback on uncommitted transactions for blocks that are being requested for consistent read by another instance.

This background process is also called Global Cache Services.
This is the name you often see back in wait events (GCS).
Default 2 LMS background processes are started.

LMON

This background process monitors the entire cluster database. LMON checks and manages instance deaths and perform recovery for the Global Cache Service//LMS.
Joining an leaving instances are managed by LMON. LMON manage also all the global resource in the RAC database. LMON register the instance/database with the node monitoring part of the cluster (CSSD).

This background process is also called Global Enqueue Monitoring.
LMON provide services are also referred to cluster group service (CGS).

LMD

This background process manage access to the blocks and global enqueues. Also global deadlock detection and remote resource request are handled by LMD. LMD also manage lock requests for GCS /LMS.

This background process is also called Global Enqueue Service Deamon. In wait events you will see GES.

DIAG

The diagnostic Daemon (DIAG) captures diagnostic data in case there is a process failure within the instance. The diag log can be used to investigate why there was a process failure.

LCK

The Lock Process is also for non-RAC environments LCK manage local noncache requests (row cache, lock requests, library locks). And it also managed shared resource requests cross instance. It keeps a list of invalid and valid lock elements. And if needed past information to the GCS.