ASM Diskgroup Compatibility

Diskgroup Attributes


ASM is a storage platform for Oracle databases ranging from 10g to the current version. So, an ASM instance on 11g can hold databases from 10g Release 1, 10g Release 2, and 11g Release 1 (and beyond). As long as the ASM version is at the same version or more than the RDBMS, it's possible to create a database on that ASM instance. So how does ASM communicate to the RDBMS instance if they are on different versions? Simple: ASM transforms the messages to suit the RDBMS version.

By default, the ASM instance can support 10g databases. But what if you want to place only 11g RDBMS on that ASM instance? The message transformation to support the version difference is not necessary. But what if there was a way to tell the ASM instance that the only databases supported is 11g Release 1? That would eliminate or at least reduce the message transformations. In Oracle Database 11g, that is possible, using the ASM Compatibility and RDBMS Compatibility diskgroup attributes.

First, let's check the current attributes of the diskgroup:

SQL> select compatibility, database_compatibility
 2  from  v$asm_diskgroup
 3  where name = 'DG1'
 4  /

COMPATIBILITY          DATABASE_COMPATIBILITY
---------------------- ----------------------
10.1.0.0.0             10.1.0.0.0


As you can see, the ASM Compatibility (shown by COMPATIBILITY) is set to 10.1.0.0.0, which means this diskgroup supports up to 10.1 ASM structure. Thus this diskgroup can have any RDBMS structure. The other column, DATABASE_COMPATIBILITY, shows the RDBMS compatibility set to 10.1. It means the ASM diskgroup DG1 can be used for any RDBMS from version 10.1.


Since you want to create only 11g ASM and RDBMS structures, there is no need to have 10g elements. To set the ASM Compatibility attribute of this diskgroup to 11.1, you issue the following statement (in the ASM instance):

SQL> alter diskgroup dg1 set attribute 'compatible.asm'='11.1';


Now, if you check the attributes of the diskgroup:
COMPATIBILITY          DATABASE_COMPATIBILITY
---------------------- ----------------------
11.1.0.0.0             10.1.0.0.0


The ASM Compatibility is set to 11.1; but RDBMS Compatibility is still set to 10.1. To change that to 11.1 as well, use:


SQL> alter diskgroup dg1 set attribute 'compatible.rdbms'='11.1';
Note one important point: the compatibility is set for the diskgroup, not for the entire ASM instance. This feature allows you to use only one ASM instance but cater to all types of database versions. Depending on which version is used, you can set the attribute appropriately and reduce inter-version communication.