Database Reference
In-Depth Information
RAC Background Processes
Each RAC database instance is a superset of a single-node instance. It has the same set of background processes
and the same memory structure, such as the
System Global Area
(SGA) and the Program Global Area (PGA). As
well as these, RAC instances also have the additional processes and memory structure that are dedicated to the GCS
processes, GES, and the global cache directory. These processes are as follows:
•
LMS: Lock Manager Server process
•
LMON: Lock Monitor processes
•
LMD: Lock Monitor daemon process
•
LCK: Lock process
•
DIAG: Diagnostic daemon
1.
LMS process: The Lock Manager Server is the Global Cache Service (GCS) process. This
process is responsible for transferring the data blocks between the RAC instances for
cache fusion requests. For a Consistent Read request, the LMS process will roll back the
block and create the Consistent Read image of the block and then transfer the block to
the requesting instance through the high-speed interconnect. It is recommended that
the number of the LMS processes is less than or equal to the number of physical CPUs.
Here the physical CPUs are the “CPU cores”; for example, for a server with two sockets
that has four cores, the number of the physical CPU is 8. By default, the number of LMS
processes is based on the number of the CPUs on the server. This number may be too high
as one LMS process may be sufficient for up to four CPUs. There are a few ways to control
the number of the LMS processes. You can modify the values for the init.ora parameter
CPU_COUNT, which will also indirectly control the number of LMS processes that will be
started during the Oracle RAC Database instance startup. The number of LMS processes
is directly controlled by the init.ora parameter GCS_SERVER_PROCESSES. For a single
CPU server, only one LMS is started. If you are consolidating multiple small databases on a
cluster environment, you may want to reduce the number of LMS processes per instance,
as there may be multiple instances of RAC databases on a single RAC node. Refer to the
Oracle support note [ID 1439551.1] “Oracle (RAC) Database Consolidation Guidelines
for Environments Using Mixed Database Versions” for detailed guidelines for setting LMS
processes for multiple databases of RAC.
2.
LMON process: The Lock Monitor process is responsible for managing the Global
Enqueue Service (GES). It is also responsible for reconfiguration of lock resources when an
instance joins or leaves the cluster and responsible for dynamic lock remastering.
3.
LMD process: The Lock Monitor daemon process is the Global Enqueue Service (GES).
The LMD process manages the incoming remote lock requests from other instances in
the cluster.
4.
LCK process: The Lock process manages non-cache fusion resource requests, such as row
cache and library cache requests. Only one LCK process (lck0) per instance.
5.
DIAG process: The Diagnostic daemon process is responsible for all the diagnostic work in
a RAC instance.
