Is this normal in 'Normal Redundancy' ?

Question

GI version: 11.2.0.4 (3-Node RAC)
OS : Oracle Linux 6.4
Storage from EMC Symmetrix VMAX
I created a diskgroup with normal redundancy as shown below.
10 LUNs of 500GB each = 5000GB
CREATE DISKGROUP DATA_DG1
NORMAL REDUNDANCY
DISK
'ORCL:DATA_D1',
'ORCL:DATA_D2',
'ORCL:DATA_D3',
'ORCL:DATA_D4',
'ORCL:DATA_D5',
'ORCL:DATA_D6',
'ORCL:DATA_D7',
'ORCL:DATA_D8',
'ORCL:DATA_D9',
'ORCL:DATA_D10'
ATTRIBUTE 'au_size' = '4M';
Diskgroup created.
Since it was normal I was expecting half of the Raw space 5000GB = 2.5 Terabyte of usable space. But, the below query output shows I got only 2.19 TeraBytes of Usable space (2249.86 GB). Around 250GB short of what I was expecting.So, where is my 250GB gone ?
SQL> select name, TYPE, TOTAL_MB/1024/1024, FREE_MB/1024/1024, USABLE_FILE_MB/1024/1024 from v$asm_diskgroup where name = 'DATA_DG1';
NAME                           TYPE   TOTAL_MB/1024/1024 FREE_MB/1024/1024 USABLE_FILE_MB/1024/1024
DATA_DG1                       NORMAL          4.8828125         4.8825531               2.19713593

Answer 1

The amount of usable file space is equal to the amount of free disk space subtracted by the amount of disk space that is required to restore redundancy after a failure divided by the redundancy level of the disk group which is two in your case.
Run:
select trunc((free_mb - required_mirror_free_mb) / 2) as useable from v$asm_diskgroup where name='DATA_DG1';
Source: http://prutser.wordpress.com/2013/01/03/demystifying-asm-required_mirror_free_mb-and-usable_file_mb/