Every time Oracle ASM or a database starts, a shared memory area called the System Global Area (SGA) is allocated and the Oracle ASM background processes are started. However, because Oracle ASM performs fewer tasks than a database, an Oracle ASM SGA is much smaller than the database SGA. The combination of background processes and the SGA is called an Oracle ASM instance. The instance represents the CPU and RAM components of a running Oracle ASM environment.
The SGA in an Oracle ASM instance is different in memory allocation and usage than the SGA in a database instance. The SGA in the Oracle ASM instance is divided into four primary areas as follows:
- Shared Pool: Used for metadata information
- Large Pool: Used for parallel operations
- Oracle ASM Cache: Used for reading and writing blocks during rebalance operations
- Free Memory: Unallocated memory available
The minimum recommended amount of memory for an Oracle ASM instance is 256 MB. Automatic memory management is enabled by default on an Oracle ASM instance and will dynamically tune the sizes of the individual SGA memory components. The amount of memory that is needed for an Oracle ASM instance will depend on the amount of disk space that is being managed by Oracle ASM.
The second part of the Oracle ASM instance is the background processes. An Oracle ASM instance can have many background processes; not all are always present. Because the Oracle ASM instance shares the same code base as an Oracle AI Database instance, all the required background processes of a database instance will exist in the Oracle ASM instance. There are required background processes and optional background processes. Some of these processes may include the following:
- CKPT: The checkpoint process
- DBWn: The database writer processes
- DIAG: The diagnosability process
- LGWR: The log writer process
- PMON: The process monitor process
- PSP0: The process spawner process
- SMON: The system monitor process
- VKTM: The virtual keeper of time process
- MMAN: The memory manager process
The preceding list of processes is not complete. There can be hundreds of database instance background processes running depending on the database options and configuration of the instance. For the Oracle ASM instance, these processes will not always perform the same tasks as they would in a database instance. For example, the LGWR process in a database instance copies change vectors from the log buffer section of the SGA to the online redo logs on disk. The Oracle ASM instance does not contain a log buffer in its SGA, nor does it use online redo logs. The LGWR process in an Oracle ASM instance copies logging information to an Oracle ASM disk group.
If Oracle ASM is clustered, additional processes related to cluster management will be running in the Oracle ASM instance. Some of these processes include the following:
- LMON: The global enqueue service monitor process
- LMDn: The global enqueue service daemons
- LMSn: The global cache service processes
- LCKn: The lock processes
Additional processes are started when ADVM volumes are configured.
- VDBG: The Volume Driver Background process forwards Oracle ASM requests to lock or unlock an extent for volume operations to the Dynamic Volume Manager driver. The VDBG is a fatal background process; termination of this process brings down the Oracle ASM instance.
- VBGn: The Volume Background processes wait for requests from the Dynamic Volume Manager driver, which need to be coordinated with the Oracle ASM instance. An example of such a request would be opening or closing an Oracle ASM volume file when the Dynamic Volume Manager driver receives an open for a volume (possibly due to a file system mount request) or close for an open volume (possibly due to a file system unmount request), respectively. Unplanned death of any of these processes does not have an effect on the Oracle ASM instance.
- VMB: Volume Membership Background coordinates cluster membership with the Oracle ASM instance.
The Oracle ASM instance uses dedicated background processes for much of its functionality.