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ASM-Automatic Storage Management – for the new oracle dba career




PURPOSE

——-

Automatic Storage Management is a file system and volume manager built into the

database kernel that allows the practical management of thousands of disk drives

with 24×7 availability.

It provides management across multiple nodes of a cluster for Oracle Real Application

Clusters (RAC) support as well as single SMP machines.

It automatically does load balancing in parallel across all available disk drives

to prevent hot spots and maximize performance, even with rapidly changing data usage

patterns.

It prevents fragmentation so that there is never a need to relocate data to reclaim

space.  Data is well balanced and striped over all disks.

It does automatic online disk space reorganization for the incremental addition or

removal of storage capacity.

It can maintain redundant copies of data to provide fault tolerance, or it can be

built on top of vendor supplied reliable storage mechanisms. Data management is done 

by selecting the desired reliability and performance characteristics for classes 

of data rather than with human interaction on a per file basis.

ASM solves many of the practical management problems of large Oracle databases.

As the size of a database server increases towards thousands of disk drives, or

tens of nodes in a cluster, the traditional techniques for management stop working. 

They do not scale efficiently, they  become too prone to human error, and they require

independent effort on every node of a cluster.

Other tasks, such as manual load balancing, become so complex as to prohibit their

application.

These problems must be solved for the reliable management of databases in the tens

or hundreds of terabytes. Oracle is uniquely positioned to solve these problems

as a result of our existing  Real Application Cluster technology.

Oracle’s control of the solution ensures it is reliable and integrated with Oracle

products.

This document is intended to give some insight into the internal workings of ASM. 

It is not a detailed design document. It should be useful for people that need to

support ASM.

Automatic Storage Management is part of the database kernel. It is linked into

$ORACLE_HOME/bin/oracle so that its code may be executed by all database processes.

One portion of the ASM code allows for the start-up of a special instance called

an ASM Instance. ASM Instances do not mount databases, but instead manage the metadata 

needed to make ASM files available to ordinary database instances.

Both ASM Instances and database instances have access to some common set of disks.

ASM Instances manage the metadata describing the layout of the ASM files. Database

instances access the contents of ASM files directly,  communicating with an ASM instance

only to get information about the layout of these files.  This requires that a second

portion of the ASM code run in the database instance, in the I/O path.

Note:

1. One and only one ASM instance required per node. So you might have multiple databases,

   but they will share the same single ASM.

2. ASM is for DATAFILE, CONTROLFILE, REDOLOG, ARCHIVELOG and SPFILE. So you can use

   CFS for common  oracle binary in RAC.

3. ASM can provide mirroring for files in a disk group.

4. In external redundancy disk groups, ASM does not mirroring.  For normal redundancy, 

   ASM 2-way mirrors files by default, but can also leave files unprotected. 

   [Unprotected  files  are  not recommended].  For high redundancy disk groups,

   ASM 3-way mirrors files.

5. Unless a user specifies an ASM alias filename during file creation, the file is

   OMF.  OMF files are deleted automatically when the higher level object (eg tablespace) 

   is dropped, whereas non-OMF files must be manually deleted. Oracle is recommending

   to use OMF.

HOW TO USE ?

————

Use DBCA to configure your ASM.

DBCA eases the configuring and creation of your database while EM provides an integrated approach

for managing both your ASM instance and database instance.

Automatic Storage Management is always installed by the Oracle Universal Installer when you

install your database software. The Database Configuration Assistant (DBCA) determines if an ASM

instance already exists, and if not, then you are given the option of creating and configuring an

ASM instance as part of database creation and configuration. If an ASM instance already exists,

then it is used instead.

DBCA also configures your instance parameter file and password file.

Steps in DBCA:

1. Choose ASM disk.

2. Create diskgroup by choosing available disk.

3. While creating ASM you have choice of mirroring for files in a disk group and

   the options are like below

   HIGH, NORMAL or EXTERNAL.

   High     -> ASM 3-way mirrors

   Normal   -> ASM 2-way mirrors

   External -> If you have already mirror disk in H/W label like EMC or another

               third party

4. dbca will create a separate instance called "+ASM" which will be in nomount stage

   to control your ASM.

5. Choose your all datafile, controlfile, redolog and spfile to your ASM volume.

Preinstall:

Here DBA will create the ASM volume, so Sysadmin should give the ownership or

proper privs to DBA.

Example in LINUX

—————-

You have two disk say "/dev/sdf" and "/dev/sdg"

Determine what those devices are bound as raw:

$ /usr/bin/raw -qa

If not:

Include devices in diskgroups by editing /etc/sysconfig/rawdevices :

      eg   /dev/raw/raw1 /dev/sdf

           /dev/raw/raw2 /dev/sdg

Set owner, group and permission on device file for each raw device:

$ chown oracle:dba /dev/raw/rawn, chmod 660 /dev/raw/rawn

Bind disk devices to raw devices:

$ service rawdevices restart

So from DBCA you can see the device "raw1" and "raw2".

After finishing of ASM volume creation, when you create a database on an ASM

volume, you should see the file details using Enterprise Manager (EM). Or you

can use V$ or DBA view to check the datafile name. Oracle recommended not to

specify the datafile name while adding datafile or creating new tablespace,

because ASM will automatically generate  OMF file.

Note: If DBA’s by mistake or intentionally choose the datafile name, dropping

of  tablespace, will not drop the datafile from ASM volume.

IMPORTANT VIEW in ASM

=====================

V$ASM_DISKGROUP

V$ASM_CLIENT

V$ASM_DISK

V$ASM_TEMPLATE

V$ASM_ALIAS

V$ASM_OPERATI

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