11g ASM Disk Group Attributes

When an instance of a database which uses ASM storage starts up, it needs to negotiate the minimum compatibility of the ASM disk groups with the corresponding ASM instance. This minimum compatibility of ASM disk groups is determined by  disk group attributes as well as by the value of the COMPATIBLE parameter of both instances.

The two most important of these attributes of an ASM disk group are compatible.asm and compatible.rdbms. Both of them can be specified at creation of the ASM disk group.

SYS AS SYSDBA @ +ASM SQL> CREATE DISK GROUP data_new3

  2  DISK '/dev/raw/raw1', '/dev/raw/raw2' ATTRIBUTE 'compatible.asm'='11.1', 'compatible.rdbms'='11.1';

They can also be changed both later on with an Alter Diskgroup statement:

SYS AS SYSDBA @ +ASM SQL> ALTER DISK GROUP fra SET ATTRIBUTE 'compatible.rdbms'='11.1';

% By omitting these parameters, a disk group is created with both compatible            attributes  set to 10.1. This means that there is no profit from the 11g New            Features for ASM!

% It is only possible to advance these attribute settings dynamically. There is no            way back.

An attempt to downgrade them results in an error:

SYS AS SYSDBA @ +ASM SQL> alter disk group fra set attribute 'compatible.rdbms'='10.1';

alter disk group fra set attribute 'compatible.rdbms'='10.1'
*
ERROR at line 1:
ORA-15032: not all alterations performed
ORA-15242: could not set attribute compatible.rdbms
ORA-15244: new compatibility setting less than current [11.1.0.0.0]

SYS AS SYSDBA @ +ASM SQL> alter disk group fra set attribute 'compatible.asm'='10.1';alter disk group fra set attribute 'compatible.asm'='10.1'

*
ERROR at line 1:
ORA-15032: not all alterations performed
ORA-15242: could not set attribute compatible.asm
ORA-15244: new compatibility setting less than current [11.1.0.0.0]

This is what the oerr interface for Linux returns as an explanation for the error:

[oracle@rhas4 ~]$ oerr ora 15244
15244, 00000, "new compatibility setting less than current [%s]"
// *Cause:  The compatibility setting was invalid. The compatibility
//          setting can only be advanced.
// *Action: Check the version number.
//

The dynamic performance view v$asm_attribute shows the attributes of ASM disk groups:

SYS AS SYSDBA @ +ASM SQL>   SELECT name, value, group_number FROM v$asm_attribute;

NAME                 VALUE                GROUP_NUMBER
-------------------- -------------------- ------------
disk_repair_time     3.6h                            1
au_size              1048576                         1
compatible.asm       11.1.0.0.0                      1
compatible.rdbms     10.1.0.0.0                      1
disk_repair_time     3.6h                            2
au_size              1048576                         2
compatible.asm       11.1.0.0.0                      2
compatible.rdbms     10.1.0.0.0                      2

There are two ASM disk groups and, as can be seen from the results of the query, there are more disk attributes. These will be covered in more depth a little bit later in this chapter.

The attributes compatible.asm and compatible.rdbms apply to both ASM and database instances. They determine which features can be used by a disk group and which ASM clients (database instances) can use the disk group.

They are furthermore determined by the initialization parameter COMPATIBLE.

The dependency is as follows:

compatible.rdbms limits the minimum compatibility of a database instance that can mount the ASM disk group. It determines the format of messages sent between the ASM instance and the DB instance which is a client for ASM. An ASM instance can manage disk groups of different compatibility attributes and its COMPATIBLE parameter must be at least the version of the highest compatible.asm attribute of any disk group it manages.

A DB instance must have its COMPATIBLE parameter set to at least the highest value of compatible.rdbms of all the disk groups which its database uses. A DB instance communicates at first startup time with the ASM instance at about the highest version of compatibility which both of them can support.

The compatibility information about the databases can be seen and their instances which use the ASM storage in v$asm_client both on the database side and on the ASM side. The code listing below shows the view from an 11g ASM instance serving both 10g and 11g clients:

SYS AS SYSDBA @ +ASM SQL> select * from v$asm_client;

GROUP_NUMBER INSTANCE_N DB_NAME  STATUS     SOFTWARE_VERSION COMPATIBLE_VERSION
------------ ---------- -------- ---------- ---------------- --------------
           1 asm11      asm11    CONNECTED    11.1.0.5.0     11.1.0.0.0
           1 asm10gR2   asm10gR2 CONNECTED    10.2.0.1.0     10.2.0.1.0

The next listing shows the view from a 10g DB instance using an 11g ASM instance:

SYS AS SYSDBA @ asm10gR2 SQL> SELECT * FROM v$asm_client;

GROUP_NUMBER INSTANCE_N DB_NAME  STATUS   SOFTWARE_VERSION   COMPATIBLE_VERSION
------------ ---------- -------- -------- ------------------ ------------------           1 +ASM       asm10gR2 CONNECTED    11.1.0.5.0         11.0.0.0.0

compatible.asm only determines the format of the persistent metadata which describe an ASM disk group. It must at least have the same value as the compatible.rdbms attribute of the disk group. It does not influence the format of files in the disk group. This is managed by the database itself.

These features include variable extent sizes, fast mirror resyncing and the capability to take a disk group offline?online as well as others which will be looked at shortly in this chapter. As has already been seen earlier, there are a few more attributes for ASM disk groups. Here they are, one by one:

1.      au_size: With this attribute, it is possible to influence the performance of ASM disk management. It is an automatic feature and does not need any manual adjusting. For more information, look at the Performance Enhancements section of this chapter

2.      disk_repair_time: specifies the amount of time before an ASM disk is dropped automatically from the disk group after offlining it. This attribute comes into play with FAST MIRROR RESYNC. For more information on this feature, refer to the respective section in this chapter.

3.      template.<template_name>.redundancy: redundancy level of template (HIGH?MIRROR?UNPROTECTED)

4.      template.<template_name>.stripe: striping unit size of template (COARSE?FINE)

% If creating a 10g database is attempted with dbca and trying to use ASM disk            groups with the attribute compatible.rdbms=11.1, an error is encountered.


Figure 2:  Error with Creating 10g Database