Oracle RAC Memory Fusion

LMS: This process is responsible for most of the work of GCS, it maintains the information of block resources in GRD, completes the transfer of data blocks between instances, and sends and receives related messages. There are multiple LMS processes in each DB instance, named Lms<x&gt, and the default number of LMS processes is calculated based on the number of CPUs in the node.

LMD: This process is mainly responsible for the management of GES related resources, GES resources mainly refers to the queue (Enqueue) resources in Oracle, because the RAC database contains many instances, each instance can operate on the same data. All require LMD processes to manage and reconcile these queue resources, with each database instance containing only one LMD process.

LCK: This process is mainly responsible for the instance lock (the meaning of the instance lock is not the instance-level lock, but the resource of this lock management is limited to the local instance) management, the lock at the instance level includes: library cache Lock,row Cachelock, etc. Instance-level locks are managed by broadcast by the LCK process, and when a process needs to hold an instance lock, the LCK process broadcasts the request to the LCK process for all remote instances, and after the remote instance receives the request, Update your information and feedback (if a process for the remote instance holds the lock in an incompatible manner, the LCK process for the instance notifies the bearer process to release the corresponding lock and then returns), and each instance contains only one LCK process by default.

GRD: One way to save and organize all the resources related to memory fusion in the shared pool and buffer cache, each of which contains GRD information, and the memory fusion information for all instances constitutes the entire GRD. The LMS and LMD processes implement GRD access and management, the LMS process is responsible for GCS resources and locks, the LMD process is responsible for ges resources and locks, and the server process acts as a client that accesses GRD, requires collaboration with the LMS and LMD processes, and completes requests sent by the client. You can assume that the server process is a GRD client, and the LMS and LMD processes are GRD server processes.

The PCM resource refers to the block of the database, and when the client accesses a database, the LMS process creates a PCM resource corresponding to that block and creates a lock structure that protects the resource.

Properties of the PCM lock

Mode: PCM lock has 3 modes, N (empty), S (shared), X (exclusive).

Role: The role of the PCM lock corresponds to the scope of the data block being modified, local (L) indicates that the corresponding block is modified only on the local instance, and global (G) indicates that the corresponding block has been modified in multiple remote instances

PI: If the corresponding block exists pi for 1, if 0 means that the corresponding block does not have PI, the lock with the role L will not exist PI,PI only the PCM lock corresponding to the role G will exist, it is a data block after multiple instances have been modified, in addition to the latest version of the data block, In more than one instance there is an intermediate version of the data file that is not DBWR written to, when the pi corresponding data block is DBWR written to the data file, the original Pi block state becomes CR. When PI is generated, the corresponding change information is written to the redo log file.

Oracle RAC Memory Fusion