ASM – Automatic Storage Management

Files Systems:

Disadvantages of Raw Devices:

1.       It supports storage of only 1 file in only 1 raw device. Hence archive redo logs files & flashback logs which are generated numerously are not suitable member for raw devices.

2.       General O/S commands like cp, ls, mv, du etc will not work in Raw Devices.

3.       Only dd (diskdump) is used for format, backup, restore the raw devices

4.       Raw devices will not support collection of I/O statistics

5.       They cannot be resize online

6.       In Linux environment, out of 15 partitions we can use only 14 for creation of raw devices, In Solaris we can use only 6 out of 7 partitions per a disk

To overcome all the disadvantages we use LVM (Logical volume manager) `

1.       It is a logical storage area which is created by collection of multiple disk partitions onto which we can create any type of file system.

2.       It supports storage of multiple file in a single volume.

3.       Online resizing is possible

4.       Supports collection of I/O statistics

5.       It improves the I/O performance & availability with the help of software level RAID techniques*.

Types of LVMs & Vendors:

LVMs	Vendors
1.       VERITAS Volume Manager 2.       Tivole Volume Manager 3.       Sun Volume Manager (SVM) 4.       ASM(from oracle 10g)	Symantec IBM Oracle SUN Oracle

ASM:

-         It is a type of LVM supported from oracle 10g and has a special type of instance. INSTANCE_TYPE=ASM. & has a small footprint of SGA with size 100-128 MB.

-          It supports for creation of logical volume known as disk groups, internally uses both strIPing and mirroring.

-          Hence it does not have any control file to mount, so its least and last stage is nomount. It has to mount the diskgroups

-          Diskgroup is a logical storage area which is created by collection of multiple disk partitions.

-         ASM supports storage of multiple database related files like control files, redo, data, archive logs, flashback logs, RMAN backup pieces, spfile etc. but it will not support the storage of static files like pfile, listener.ora. tnsnames.ora sqlnet.ora etc.

-         From 11.2 onwards, by using ADVM (ASM dynamic volume manager) & ACFS (ASM cluster file system) we can store static files also.

Note: Sometimes ASM instance may contain large pool also.

-          1 ASM instance will support creation of multiple disk groups and will provide services to multiple clients.

ASM Clients: These are general DB instances which are dependent on ASM instance in order to access the diskgroups.

ASM Instance Background processes:

RBAL – Rebalance Master: It is responsible for managing and coordinating the disk group activities and also responsible for generating the plans for even distribution of ASM instance (extends) for better load balancing whenever a new disk is added and removed.

ARBn – ASM Rebalancer: It is a slave process of RBAL background process and it is responsible for actual load balancing of ASM Disks.

ASMB – ASM Background: It is responsible for successful establishment of communication channel between ASM instance & ASM clients.

GMON – Global Monitor: It is responsible for coordinating the disk group activities whenever a disk group becomes offline or drop.

KATE – Konductor for ASM Temporary Errand: It is responsible for making online for disk groups.

ASM client Background Processes:

RBAL – Rebalance Master: It is responsible for successful opening and closing the diskgroups whenever a read or write operations occur.

PZ9X: It is responsible for gathering the dynamic views information globally across all the instances of database.

ASM related dynamic views:

In RAC environment, all the dynamic views start with gv$ , in non-RAC g$

1.       gv$ asm_disk

2.       gv$ asm_diskgroups

3.       gv$ asm_io_stat

4.       gv$ asm_clients

5.       gv$ asm_template (total 19 views)

  

ASM in RAC Environment: