Internal Workings of the Oracle RAC System
In 9i RAC, we no longer talk about DLM,PCM, non-PCM, Lock Monitor,etc.Most of the functionality is replaced in Oracle 10g or performed in the name of Global Cache Services. A lock is now treated as a held resource. The background processes in the previous versions still exist but serve different functions.
RAC Instances and Processes
RAC is a multi-instance database. Multiple instances access the same database concurrently. There is not much of a difference in terms of structure between a RAC instance and a stand-alone Oracle instance .Besides all the usual Oracle Processes like PMON,SMON,LGWR, and DBWR,there are many special processes spawned to coordinate inter-instance communication and to facilitate resource sharing among nodes in a cluster. Because of the inter-instance buffer movement and the new set of blocks, called Past Image Blocks (to preserve data integrity), additional resources from the SGA are used.
LMON: THE Global Enqueue Service Monitor(LMON) monitors the entire cluster to manage global enqueues and resources. LMON manages instance and process expirations and the associated recovery for the Global Cache Service.
LMD: The Global Enqueue Service Daemon(LMD) is the lock agent process that manages enqueue manager service requests for Global Cache Service enqueues to control access to global enqueues and resource. The LMD process also handles deadlock detection and remote enqueue request.
LMSn: These Global Cache Service processes(LMSn) are processes for the Global Cache Service(GCS). RAC software provides for up to ten Global Cache Service processes. The number of LMSn varies depending on the among of messaging traffic among nodes in the cluster. The LMSn processes to these things:
·Handle blocking interrupts from the remote instance for Global Cache Service resources.
·Manage resource requests and cross-instance call operations for shared resources.
·Build a list of invalid block elements and validate lock elements during recovery.
·Handle global lock deadlock detection and monitor lock conversion timeouts.
LCK process: Manages global enqueue requests and cross-instance broadcast.
DIAG: The Diagnosability Daemon monitors the health of the instance. It captures data for instance process failures.
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