I/O Activity Balance Across Disks in ASM

Balanced Oracle ASM Disk Groups

Check cluster name using below command:

[grid@rac1 ~]$ cemutlo -n
rac-cluster
[grid@rac1 ~]$

• Make sure that all the disks in the same Oracle ASM disk group are the same size. Oracle ASM writes in a round-robin fashion to the Oracle ASM disk group disks; therefore, small Oracle ASM disks will be full faster than larger disks, which results in unbalanced I/O activity across the Oracle ASM disks.

• All disks in the same Oracle ASM disk group should have the same performance characteristics

Use below query to find out ASM disks are blanced or not :-

SELECT dg.group_number "GROUP#",
       dg.name,
       DECODE (total_dg.total_io, 0, 100, 100 * (DECODE (SIGN (1 - df.sum_io / total_dg.total_io), -1, 0, (1 - df.sum_io / total_dg.total_io)))) "IO_BALANCED"
  FROM (SELECT d.group_number group_number,
                 SUM (ABS ((d.reads + d.writes) - tot.avg_io)) sum_io
            FROM v$asm_disk_stat d,
                 (SELECT group_number,
                           SUM (reads) + SUM (writes),
                           DECODE (COUNT (*), 0, 0, (SUM (reads) + SUM (writes)) / COUNT (*)) avg_io
                      FROM v$asm_disk_stat
                     WHERE header_status = 'MEMBER'
                  GROUP BY group_number) tot
           WHERE header_status = 'MEMBER' AND tot.group_number = d.group_number
        GROUP BY d.group_number) df,
       (SELECT group_number,
                 SUM (reads) + SUM (writes) total_io
            FROM v$asm_disk_stat
           WHERE header_status = 'MEMBER'
        GROUP BY group_number) total_dg,
        V$ASM_DISKGROUP dg
 WHERE df.group_number = total_dg.group_number
 AND df.group_number = dg.group_number;

In above figure shows an example of my environment where there are two disk groups in the Oracle ASM instance: DATA1 and DATA2. As you can see, the I/O activity is distributed equally across the Oracle ASM disks in each disk group. The conclusion here is that the disk groups (DATA1  and DATA2) are totally balanced.




For more explanation check below link:

https://community.oracle.com/docs/DOC-995178