Developers Guide

Components of ASM
11 Dell EMC SC Series Storage with Oracle ASM | CML1111
3 Components of ASM
3.1 ASM instance
Starting with Oracle 10g and above, there are two types of instances: database and ASM. The ASM instance,
which is generally named +ASM, is started with the INSTANCE_TYPE=ASM asm init.ora parameter. This
parameter signals the Oracle initialization routine to start an ASM instance and not a standard database
instance. Unlike the standard database instance, the ASM instance contains no physical files and only
requires a few init.ora parameters for startup. Upon startup, an ASM instance will spawn all the basic
background processes, plus some new ones that are specific to the operation of ASM. For a list and
description of the background processes, see the appendices of this document.
3.2 ASMLib
As mentioned in the prior section, Oracle developed two optional storage management API interfaces for
block-device management, discovery, and provisioning, one of which is ASMLib. ASMLib provides storage
vendors the ability to supply a plug-in library to extend storage-related features and exploit storage array
capabilities when using ASM. It is an add-on module that simplifies the management and discovery of ASM
disks and provides an alternative to the standard operating system interface for ASM to identify and access
block devices.
ASMLib is not required to run ASM, but if ASMLib is not used, either ASMFD or native OS methods (like
UDEV rules) must be used to manage the ASM block devices. ASMLib also allows a database to use more
efficient access to the logical volume groups it is using.
If ASMLib is used, the user and group permissions will be set correctly on the devices under
/dev/oracleasm/disks/*. The values of user and group are configured while running /etc/init.d/oracleasm
configure.
3.3 Disk groups
Disk groups are the primary component of ASM. Once the ASM disk devices are discovered, a disk group can
be created that will encapsulate one or more of the ASM disks. Once the disk groups are created, they can be
specified as the default location for database files created in the database. Files written on ASM disks are
ASM files and can have automatically generated filenames. A disk group is the highest-level data structure in
ASM and is comparable to an LVM volume group. There are some differentiators between typical LVM
volume groups and ASM disk groups.
• An ASM file system layer is implicitly created within a disk group. This file system is transparent to
users and only accessible through ASM, interfacing databases, and the ASM command-line tool.
• There are inherent automatic file-level striping and mirroring capabilities. A database file created
within an ASM disk group will have its file extents (not database extents) distributed equally across all
online disks in the disk group, which provides an even I/O load.
Oracle recommends that generally no more than two disk groups be maintained and managed per single
ASM instance or RAC cluster. However, it is highly recommended to evaluate this recommendation and
create additional disk groups as necessary. For a discussion on when additional disk groups might be
necessary or desirable, see the discussion on snapshot profiles in the document, Dell EMC SC Series Arrays
and Oracle. Data that can be categorized with different storage characteristics can be stored in distinct disk