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Friday, March 2, 2012

RAC Background Processes.

Lock Monitor(LMON) Process.

LMON is responsible for monitoring all instances in a cluster
for the detection of failed instances.

Once a failed Instance is detected it facilitates in the 
recovery of global locks held by that instance.

It is also responsible for reconfiguration of locks and other
resources when instances leave or are added to the cluster.
This dynamic reconfiguration is done in realtime.


Lock Manager Server(LMS) process.

The Primary job of LMS process is to transport blocks across the
nodes.

For example if a node requests consistent-read of a block, The LMS process 
makes a Consistent-Read image of the block from another node with the help of undo segments
and then transports the blocks through the network to the node who requested it.

Earlier before 10g whenever a read consistent image of a block was required
by a node, the other node which held the block in exclusive mode would flush the block
to the disk and then the node which needed the block would get it from the disk.


Lock Manager Daemon (LMD) process.

The LMD process basically acts as a broker to LMS process by 
sending requests for resources to a queue that is handled by the LMS process.

These requests are placed by the global cache service in order to
keep the block buffers consistent across all the instances.

The other responsibility of LMD is of detection and resolution of global deadlocks,
along with monitoring of lock timeouts in the global environment.

Lock (LCK) process.

The LCK process is similar to LMD process, but it handles requests for all global 
resources excluding requests for database block buffers.


DIAG (Diagnostic Daemon).

This process is available from oracle 10g onwards.

It regularly monitors the health of the instance.
Checks for instance hangs and deadlocks.
It captures diagnostic data for instance and process failures. 

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