Oracle ASM的AU（Allocation Units）

Allocation Units

Every Oracle ASM disk is divided into allocation units (AU). An allocation unit is the fundamental unit of allocation within a disk group. A file extent consists of one or more allocation units. An Oracle ASM file consists of one or more file extents.

When you create a disk group, you can set the Oracle ASM allocation unit size with the 

AU_SIZE

 disk
group attribute. The values can be 1, 2, 4, 8, 16, 32, or 64 MB, depending on the specific disk group compatibility level. Larger AU sizes typically provide performance advantages for data warehouse applications that use large sequential reads.

For information about specifying the allocation unit size for a disk group, see "Specifying the Allocation Unit Size". For information about disk group compatibility attributes, see "Disk
Group Compatibility".

Specifying the Allocation Unit Size

Oracle recommends that the allocation unit (AU) size for a disk group be set to 4 megabytes (MB). In addition to this AU size recommendation, the operating system (OS) I/O size should
be set to the largest possible size.

Some benefits of a 4 MB allocation unit are:

Increased I/O through the I/O subsystem if the I/O size is increased to the AU size.

Reduced SGA size to manage the extent maps in the database instance.

Faster datafile initialization if the I/O size is increased to the AU size.

Increased file size limits.

Reduced database open time.

The allocation unit size is specified with the disk group attribute 

AU_SIZE

. The AU size cannot be altered after a disk group is created. Example
4-1 shows how the 

AU_SIZE

 is specified with the 

CREATE

 

DISKGROUP

 SQL statement.
{

Example  Creating the DATA disk group

{

CREATE DISKGROUP data NORMAL REDUNDANCY

  FAILGROUP controller1 DISK

    '/devices/diska1' NAME diska1,

    '/devices/diska2' NAME diska2,

    '/devices/diska3' NAME diska3,

    '/devices/diska4' NAME diska4

  FAILGROUP controller2 DISK

    '/devices/diskb1' NAME diskb1,

    '/devices/diskb2' NAME diskb2,

    '/devices/diskb3' NAME diskb3,

    '/devices/diskb4' NAME diskb4

  ATTRIBUTE 'au_size'='4M',

    'compatible.asm' = '11.2', 

    'compatible.rdbms' = '11.2',

    'compatible.advm' = '11.2';

}

Disk Group Compatibility

This section describes disk group compatibility under the following topics:

Overview of Disk Group Compatibility

Disk Group Compatibility Attributes

Setting Disk Group Compatibility Attributes

Considerations When Setting Disk Group Compatibility
in Replicated Environments

Overview of Disk Group Compatibility

Advancing the disk group compatibility settings enables you to use the new Oracle ASM features that are available in a later release. For example, a disk
group with the disk group compatibility attributes set to 11.2 can take advantage of new Oracle 11g release 2 (11.2) features, such as Oracle ASM volumes in disk groups and Oracle ASM File Access Control.
See Table 4-3 for the features enabled for combinations of compatibility attribute settings.

The disk group compatibility feature also enables environments to interoperate when they use disk groups from both Oracle Database 10g and Oracle Database 11g.
For example, disk group compatibility settings that are set to Oracle Database 10g enable an Oracle 10g client to access a disk group created with Oracle
ASM 11g.

The disk group attributes that determine compatibility are 

COMPATIBLE.ASM

, 

COMPATIBLE.RDBMS

. and 

COMPATIBLE.ADVM

.
The 

COMPATIBLE.ASM

 and

COMPATIBLE.RDBMS

 attribute settings determine the minimum Oracle Database software version numbers that a system can use for Oracle
ASM and the database instance types respectively. For example, if the Oracle ASM compatibility setting is 

11.2

, and RDBMS compatibility is set to 11.1, then the Oracle ASM software version must be at least 

11.2

,
and the Oracle Database client software version must be at least 

11.1

. The 

COMPATIBLE.ADVM

attribute determines whether the Oracle ASM Dynamic Volume Manager
feature can create an volume in a disk group.

When you create a disk group, you can specify the disk group compatibility attribute settings in the 

CREATE

 

DISKGROUP

 SQL statement.
The 

ALTER

DISKGROUP

 SQL statement can update the compatible attribute settings for existing disk groups. If not specified when using the 

CREATE

 

DISKGROUP

 SQL
statement, 

10.1

 is the default setting for both the 

COMPATIBLE.ASM

 and 

COMPATIBLE.RDBMS

 attributes for
Oracle ASM in Oracle Database 11g. The

COMPATIBLE.ADVM

 attribute is empty if it is not set. See Table
4-2 for examples of valid combinations of compatible attribute settings.

You can set and update disk group attributes with the ASMCMD 

setattr

 command. For information about the ASMCMD 

setattr

 command,
see"setattr".

Notes:

The disk group compatibility settings determine whether your environment can use the latest Oracle ASM features.

The disk group compatibility settings can only be advanced; you cannot revert to a lower compatibility setting. See "Reverting
Disk Group Compatibility".

The 

COMPATIBLE.ASM

 attribute must be advanced before advancing other disk group compatibility attributes and its value must be greater than or equal to the value of other disk group
compatibility attributes.

setattr

Purpose
Sets the attributes for an Oracle ASM disk group.

Syntax and Description

setattr

 

-G

 

diskgroup

 

attribute_name

 

attribute_value

Table 12-46 lists the syntax options for the 

setattr

 command.

Table 12-46 Options for the setattr command

Option	Description
-G   diskgroup	Disk group name.
attribute_name	Name of the attribute.
attribute_value	Value of the attribute.

The 

COMPATIBLE.ASM

 attribute must be advanced before advancing other disk group compatibility attributes and its value must be greater than or equal to the value of other disk group
compatibility attributes.

For information about disk group attributes, see "Disk Group Attributes".

Example
The following are examples of the 

setattr

 command. The first example sets the disk group attribute 

COMPATIBLE.ASM

 to 

11.2

 for
the 

data

 disk group. The second example sets the disk group attribute 

COMPATIBLE.RDBMS

 to 

11.2

 for the 

data

 disk
group.

Example 12-50 Using the ASMCMD setattr command

ASMCMD [+] > setattr -G data compatible.asm 11.2.0.0.0

ASMCMD [+] > setattr -G data compatible.rdbms 11.2.0.0.0

umount

Purpose
Dismounts a disk group.

Syntax and Description

umount

 { 

-a

 | [

-f

] 

diskgroup

 }
Table 12-47 lists the syntax options for the 

umount

 command.

Table 12-47 Options for the umount command

Option	Description
diskgroup	Name of the disk group.
-a	Dismounts all mounted disk groups. These disk groups are listed in the output of the  V$ASM_DISKGROUP view.
-f	Forces the dismount operation.

Example
The following are examples of the 

umount

 command. The first example dismounts all disk groups mounted on the Oracle ASM instance. The second example forces the dismount of the 

data

 disk
group.

Example 12-51 Using the ASMCMD umount command

ASMCMD [+] > umount -a

ASMCMD [+] > umount -f data

OCP 题库考核

Q128. What is the default AU size of an ASM diskgroup? What is the maximum AU size in an ASM disk group?

A. 100KB default, 10TB maximum

B. 256KB default, 1024MB maximum

C. 10MB default, 126PB maximum

D. 64KB default, 1EB maximum

E. 1MB default, 64MB maximum

 

Answer: E