How To Check if ASM Diskgroup is Not Overbooked

The query below can be used in order to recognize overbooked ASM diskgroups.
The value of column real_avail_gb should be >= 0

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-- total_gb      : Physical size of the disk group in GB

-- usable_gb     : Corresponds to usable_file_mb in GB

--                 i.e. Amount of free space that can be safely

--                 utilized 

--                 taking mirroring into account and yet be able to

--                 restore redundancy after a disk failure

-- df_maxsizes_gb: Sum of the datafiles max sizes in GB

-- real_avail_gb : (total_gb - real_avail_gb)

--                 Shows if there would be enough space on the

--                 diskgroup even if all files would grow up to their

--                 maxsize

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SELECT name, total_gb, usable_gb, df_maxsizes_gb,

       (total_gb - df_maxsizes_gb) real_avail_gb

  FROM

  (

    SELECT name, total_mb/1024 total_GB,

           -- free_mb/1024 free_gb,

           usable_file_mb/1024 usable_gb,

           DF.maxsize_gb df_maxsizes_gb

      FROM v$asm_diskgroup,

           ( 

             SELECT SUBSTR(file_name, 2, 

                          INSTR(file_name, '/', 1) - 2) disk_group,

                   SUM(DECODE(maxbytes,0,bytes/(1024*1024*1024),

                              maxbytes/(1024*1024*1024))

                      ) maxsize_gb

               FROM sys.dba_data_files

              GROUP BY SUBSTR(file_name, 2, INSTR(file_name, '/', 1) - 2)

           ) DF

     WHERE DF.disk_group = name

  )

 ORDER BY 1;

Example output:

NAME	TOTAL_GB	USABLE_GB	DF_MAXSIZES_GB	REAL_AVAIL_GB
U01	1600	236.484375	1504.064453125	95.935546875

The query from above was built based on the following queries

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-- Quering the ASM diskgroups directly from the DB

-- (no need to logon to the ASM DB instance)

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SELECT name,

       total_mb/1024 total_GB,

       free_mb/1024 free_gb,

       usable_file_mb/1024 usable_gb

  FROM v$asm_diskgroup;

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-- Tablespace MaxSizes by ASM diskgroup

-- Precondition: Diskgroup starts with +.

-- For example: file_name="+U01/folder/file.ext" => disk_group="U01" 

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SELECT SUBSTR(file_name, 2, INSTR(file_name, '/', 1) - 2) disk_group,

       tablespace_name,

       SUM(DECODE(maxbytes,0,bytes/(1024*1024*1024),

           maxbytes/(1024*1024*1024))

       ) maxsize_gb

  FROM sys.dba_data_files

 GROUP BY SUBSTR(file_name, 2, INSTR(file_name, '/', 1) - 2), tablespace_name

 ORDER BY 1;