ASM Components ASM Instance

Every time ASM or a database is started, a shared memory area called the System Global Area (SGA) is allocated and Oracle ASM background processes are started. 

The combination of the background processes and the SGA is called an Oracle ASM instance. The instance represents the CPU and RAM components of a running ASM environment.

The SGA in an ASM instance is different in memory allocation and usage than the SGA in a database instance. The SGA in the ASM instance is divided into four primary areas as follows:

• Shared Pool: Used for metadata information
• Large Pool: Used for parallel operations
• ASM Cache: Used for reading and writing blocks during rebalance operations
• Free Memory: Unallocated memory available

The minimum recommended amount of memory for an ASM instance is 256 MB. Automatic memory management is enabled by default on an ASM instance and will dynamically tune the sizes of the individual SGA memory components. The amount of memory that is needed for an ASM instance will depend on the amount of disk space being managed by ASM.

The second part of the ASM instance is the background processes. An ASM instance can have many background processes; not all are always present.

• ARCn: The archiver processes
• CKPT: The checkpoint process
• DBWn: The database writer processes
• DIAG: The diagnosability process
• Jnnn: Job queue processes
• LGWR: The log writer process
• PMON: The process monitor process
• PSP0: The process spawner process
• QMNn: The queue monitor processes
• RECO: The recoverer 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 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 ASM instance does not contain a log buffer in its SGA, nor does it use online redo logs. The LGWR process in an ASM instance copies logging information to an ASM disk group.

If ASM is clustered, additional processes related to cluster management will be running in the 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 ASM requests to lock or unlock an extent for volume operations to the Dynamic Volume Manager driver. The VDBG is a fatal background process so the termination of this process brings down the ASM instance.

• VBGn: Volume Background processes wait for requests from the Dynamic Volume Manager driver, which need to be coordinated with the ASM instance. An example of such a request would be opening or closing an 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). The unplanned death of any of these processes does not have an effect on the ASM instance.

• VMB: Volume Membership Background (VMB) coordinates cluster membership with the ASM instance.

ASM Components: Configuration Files

ASM instance configuration files include: 

• SPFILE
• –orapw+ASM, which is the binary password file used for remote authentication to the ASM instance

Node listener configuration files include:

• –listener.ora, a text file that defines the node listener
• –sqlnet.ora, an optional text file that provides additional listener options

Other miscellaneous text configuration files include:

• –/etc/oratab, which lists all the instances on the host machine
• –/etc/oraInst.loc, which defines the Oracle inventory directory  

Oracle Clusterware is responsible for:

• Starting and stopping ASM instances
• Starting and stopping dependent database instances
• Mounting and dismounting disk groups
• Mounting and dismounting ACFS volumes 

ASM Components: ASM Disk Group

The ASM disk group is the fundamental object that ASM manages.It: 

• Consists of one or more ASM disks that provide space
• Includes self-contained metadata and logging information for management of space within each disk group
• Is the basis for storage of ASM files
• Supports three disk group redundancy levels:
• Normal defaults to internal two-way mirroring of ASM files.
• High defaults to internal three-way mirroring of ASM files.
• External uses no ASM mirroring and relies on external disk hardware or redundant array of inexpensive disks (RAID) to provide redundancy.
• Supports ASM files from multiple databases

ASM Disk Group: Failure Groups 

A failure group is a subset of the disks in a disk group, which could fail at the same time because of shared hardware.

• Failure groups enable the mirroring of metadata and user data.
• The default failure group creation puts every disk in its own failure group.
• Multiple disks can be placed in a singe failure group at disk group creation.
• Failure groups apply only to normal and high redundancy disk groups.
• A normal redundancy disk group requires at least two failure groups to implement two-way mirroring of files.
• A high redundancy disk group requires at least three failure groups to implement three-way mirroring of files.

ASM Components: ASM Disks

ASM disks are the storage devices provisioned to ASM disk groups.

• Are formed from five sources as follows:
• A disk or partition from a storage array
• An entire physical disk or partitions of a physical disk
• Logical volumes (LV) or logical units (LUN)
• Network-Attached Files (NFS)
• Exadata grid disk
• Are named when added to a disk group using a different name than the operating system device name
• May use different operating system device names on different nodes in a cluster for the same ASM disk
• Are divided into allocation units (AU) with sizes 1, 2, 4, 8, 16, 32, or 64 MB allowed

ASM Components: ASM Files

Control files	Flashback logs	Data Pump dump sets
Data files	DB SPFILE	Data Guard configuration
Temporary data files	RMAN backup sets	Change tracking bitmaps
Online redo logs	RMAN data file copies	OCR files
Archive logs	Transport data files	ASM SPFILE