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 theAU_SIZEdisk 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 attributeAU_SIZE. The AU size cannot be altered after a disk group is created.Example 4-1shows how theAU_SIZEis specified with theCREATEDISKGROUPSQL 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 11grelease 2 (11.2) features, such as Oracle ASM volumes in disk groups and Oracle ASM File Access Control. SeeTable 4-3for 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 10gand Oracle Database 11g. For example, disk group compatibility settings that are set to Oracle Database 10genable an Oracle 10gclient to access a disk group created with Oracle ASM 11g.

The disk group attributes that determine compatibility areCOMPATIBLE.ASM,COMPATIBLE.RDBMS. andCOMPATIBLE.ADVM. TheCOMPATIBLE.ASMandCOMPATIBLE.RDBMSattribute 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 is11.2, and RDBMS compatibility is set to 11.1, then the Oracle ASM software version must be at least11.2, and the Oracle Database client software version must be at least11.1. TheCOMPATIBLE.ADVMattribute 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 theCREATEDISKGROUPSQL statement. TheALTERDISKGROUPSQL statement can update the compatible attribute settings for existing disk groups. If not specified when using theCREATEDISKGROUPSQL statement,10.1is the default setting for both theCOMPATIBLE.ASMandCOMPATIBLE.RDBMSattributes for Oracle ASM in Oracle Database 11g. TheCOMPATIBLE.ADVMattribute is empty if it is not set. SeeTable 4-2for examples of valid combinations of compatible attribute settings.

You can set and update disk group attributes with the ASMCMDsetattrcommand. For information about the ASMCMDsetattrcommand, 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".

• TheCOMPATIBLE.ASMattribute 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-Gdiskgroupattribute_nameattribute_value

Table 12-46lists the syntax options for thesetattrcommand.

Table 12-46 Options for the setattr command

Option Description

-Gdiskgroup	Disk group name.
attribute_name	Name of the attribute.
attribute_value	Value of the attribute.

TheCOMPATIBLE.ASMattribute 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 thesetattrcommand. The first example sets the disk group attributeCOMPATIBLE.ASMto11.2for thedatadisk group. The second example sets the disk group attributeCOMPATIBLE.RDBMSto11.2for thedatadisk 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 12-47lists the syntax options for theumountcommand.

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 theV$ASM_DISKGROUPview.
-f	Forces the dismount operation.

Example

The following are examples of theumountcommand. The first example dismounts all disk groups mounted on the Oracle ASM instance. The second example forces the dismount of thedatadisk 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