Tuning the Storage Subsystem - Expert Oracle RAC Performance Diagnostics and Tuning - page 369

ASM Architecture
ASM is the storage-management solution from Oracle. Like a database instance, ASM has an instance of its own.
Starting with Oracle Database Version 12c Release 1, with the flex-ASM option, only three instances of ASM are required
to support a multi-node RAC configuration. Being an instance itself, ASM also has a set of background and foreground
processes. To understand how all the various components related to ASM work together, let us examine Figure 12-10 .

LMSn

ASMB

+ASM1

+ASM2

ARBn

1

SSKY1 LMSn

DBWR

4

SSKY2

6

ARBn

RBAL

ASMB

7

EVM CSS

EVM

RBAL

CSS

2 3

CRS CRS

DBWR

SAN switch

5

prddb

OCR

Vote Disk

AA_00 AB_01 AC_02 AD_03 AE_04 AF_05 AG_06 AH_07

Figure 12-10. ASM process flow

1. When the diskgroup is created, the ASM instance loads this information into the SGA of

the ASM instance; it is stored on each disk header within the diskgroup.

At the start of the ASM instance, the ASM Rebalance Master Process ( RBAL ) is a
background process that will discover, open, and mount all ASM diskgroups on the
respective nodes. After the diskgroup is mounted (on instance startup) ASM registers
the diskgroup name, the instance name, and the corresponding Oracle Home path with
Cluster Synchronization Service (CSS).

2.

The ASM Background Process ( ASMB ) on the RDBMS instance will verify with the CSS if
any diskgroups are assigned to it and then obtain the connect string to the ASM instance.
During the RDBMS instance startup process, this information is used by the RDBMS
instance to retrieve all diskgroup information from the ASM instance.

3.

When a user adds a new disk to an existing diskgroup, the RBAL background process will
create a map to reorganize the extents in the diskgroup. The RBAL process will then send
a message to the ARBn background process to reorganize the extents. The number of ARBn
background processes started is based on the parameter ASM_POWER_LIMIT .

4.