REQUIRED_MIRROR_FREE_MB is non-zero when add 2 disk in ASM normal redundancy diskgroup

Hi all,

I have created DATA diskgroup in ASM with parameter: 2 disk (70GB per disk), normal redundancy.

GROUP_NUMBER NAME                           STATE            TYPE             TOTAL_MB    FREE_MB      REQUIRED_MIRROR_FREE_MB USABLE_FILE_MB OFFLINE_DISKS

------------                    ------------------------------ -----------           ------           ----------           ----------           -----------------------                               --------------                -------------

           1                     DATA                           MOUNTED     NORMAL     138980          131546                       0                                              65773                       0

DISK_GROUP_NAME                DISK_FILE_PATH                 DISK_FILE_NAME                 DISK_FILE_FAIL_GROUP

------------------------------                ----------------------------- ------------------------------ ------------------------------

DATA                                          /dev/rdsk/c4t0d1s6             DATA_0001                      DATA_0001

DATA                                          /dev/rdsk/c4t0d0s6             DATA_0000                      DATA_0000

[CANDIDATE]                             /dev/rdsk/c4t0d3s6

[CANDIDATE]                             /dev/rdsk/c4t0d4s6

After that, i added one disk (70GB) into DATA diskgroup SQL>ALTER DISKGROUP DATA ADD DISK '/dev/rdsk/c4t0d3s6'