目前磁盘组可用空间 FREE_MB/2

冗余模式空间计算

REQUIRED_MIRROR_FREE_MB值代表了如果ASM磁盘或ASM磁盘组出现了故障，要重新满足冗余度的定义，要求有多少的空闲空间。USABLE_FILE_MB的值代表了在满足冗余度之后，还有多少的剩余空间可以用。

满足冗余模式之后空间计算公式

(FREE_MB - REQUIRED_MIRROR_FREE_MB) / 2 = USABLE_FILE_MB
(2504-0)/2= 1252

State    Type    Rebal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Voting_files  Name
MOUNTED  NORMAL  N         512   4096  1048576      4092     2504                0            1252              0             N  DG1/

Usable_file_MB 负数？

当Free_MB < Req_mir_free_MB,那么Usable_file_MB 肯定是负值，这种情况下磁盘组如果有一个磁盘失效,是不能有效恢复成redundancy的.即处在"不安全"的状态下.

（656-2046）/2 = -695

ASMCMD> lsdg
State    Type    Rebal  Sector  Block       AU  Total_MB  Free_MB  Req_mir_free_MB  Usable_file_MB  Offline_disks  Voting_files  Name
MOUNTED  NORMAL  N         512   4096  1048576      8184      656             2046            -695              0             N  DG1/

Req_mir_free_MB 数值如何确定？

磁盘组容量最的磁盘确定该数值大小。

The value is the total raw space for all of the disks in the largest failure group. The largest failure group is the one with the largest total raw capacity. For example, if each disk is in its own failure group, then the value would be the size of the largest capacity disk.

参考手册

Automatic Storage Management Administrator's Guide
4 Administering Oracle ASM Disk Groups

https://docs.oracle.com/cd/E11882_01/server.112/e18951/asmdiskgrps.htm#OSTMG137

REQUIRED_MIRROR_FREE_MB （Req_mir_free_MB）/ USABLE_FILE_MB