Support of Oracle RAC ASM with SGeRAC, January 2008

HP Availability Clusters Solutions Lab 1/25/2008 Page 4

• It has a set of foreground (asm1.. asmN) and background processes accessing a shared

memory segment. The background processes include processes that resemble those of the

database instance such as pmon, dbwr, .. as well as some additional ASM-specific

processes such as rbal to perform disk group rebalancing.

• It uses its foreground processes to interact with clients such as database instances and db

utilities such as sqlplus.

For a database using ASM-managed storage, the database instance has, in addition to the standard

foreground (ora1,.. oraN) and background processes (pmon, dbwr, ..), some ASM specific processes

such as rbal and asmb for communication with the ASM instance.

Figure 2. Single Node ASM Environment

When a database instance starts up, it opens the database files, and in doing so, the instance

acquires file extent maps from the ASM instance for each file. These maps are used by the database

instance to translate I/O file addresses to device block addresses.

To provide the extent maps, the ASM instance scans the ASM metadata on the devices that it

manages. When the metadata changes, for example due to device addition or deletion, the ASM

instance updates the database instance with the new information.

Thus the database instance does not have to communicate with the ASM instance for every I/O read

or write; it uses the copy of file extent maps that it has cached to generate one or more corresponding

I/O requests directly to the operating system.

Oracle DBs in ASM

Disk Groups

DB Mana

ement Utilities

Enterprise

Manager (EM)

DBCA

sqlplus

pmon

lgwr

dbwr

asmb

ora1 ora

pmon lgwr

dbwr

rbal

asm1 asm

Get DB File Extent Maps

-Read/Write Disk Group Headers

-Rebalance Disk Group Data

View/Create/Modify Disk Groups

Read/Write DB Files

DB Instances

ASM Instance