The following is from a former colleague, worth mentioning.

The ASM targets in OEM-GC give the next active warnings:

Metric: Disk Group Percent Imbalance
Message: Disk Group SYSTEMDG requires rebalance because the space usage imbalance between disks is high.

Metric: Disk Size Imbalance (%)
Message: Disk Group SYSTEMDG has failure groups with disks of different sizes which may lead to suboptimal space usage. Changing the configuration may alleviate this problem.

Reason for this warnings: created a SYSTEMDG diskgroup on two SAN’s and one NFS-node:
o    S1_SYSTEMDG_001    –  ASM disk on SAN1        (4.9Gb)
o    S1_SYSTEMDG_002    –  ASM disk on SAN1        (4.9Gb)
o    S2_SYSTEMDG_001    –  ASM disk on SAN2        (4.9Gb)
o    S2_SYSTEMDG_002    –  ASM disk on SAN2         (4.9Gb)
o    SYSTEMDG_0004    –  NFSfile on node pdcc1b –    (0.49Gb)
·    The  5th disk has been added as an extra Quorum (voting) disk.

Preferred solution: raise de NFS voting disk to 4.9Gb, and create a new voting file on the NFS voting server pdcc1b like this:

[grid@pdcc1b ~]$ cd /opt/oracle/vote/
[grid@pdcc1b vote]$ dd if=/dev/zero of=vote_file_5020 bs=1k count=5020k
Ø    Connect the new voting file to SYSTEMDG
SQL> ALTER DISKGROUP SYSTEMDG ADD QUORUM DISK ‘/opt/oracle/vote/vote_file_5020’ NAME NFS_SYSTEMDG_VOTE SIZE 5020 M
Ø    Remove the old voting file from the SYSTEMDG
SQL> ALTER DISKGROUP SYSTEMDG DROP DISK SYSTEMDG_0004