From time to time, it’s useful to know the total allocated size of a database at OS level, how much of it has been used and what the maximum total size the database can grow to at OS level, see blog post:
How to obtain the Database Total Used, Allocated and Max Size

However, it’s also good to know the historical size.  The below query will give you this from the Oracle Enterprise Manager (OEM) repository:

SELECT Database,
Month_Date,
round(sum(decode(metric_column, 'spaceUsed', maximum))/1024/1024, 3) Used_Size_Tb,
round(sum(decode(metric_column, 'spaceAllocated', maximum))/1024/1024, 3) Allocated_Size_Tb
FROM
(
SELECT target_name Database, trunc(rollup_timestamp, 'MONTH') Month_Date, key_value TB, metric_column, round(max(maximum),0) maximum
FROM mgmt$metric_daily
WHERE target_type = 'rac_database'
and metric_name = 'tbspAllocation'
and metric_column in ('spaceAllocated', 'spaceUsed')
and target_name in ('VERS')
GROUP BY target_name, key_value, trunc(rollup_timestamp, 'MONTH'), metric_column
)
GROUP BY Database, Month_Date
ORDER BY Database, Month_Date
/

Output:

DATABASE   MONTH_DAT USED_SIZE_TB ALLOCATED_SIZE_TB
---------- --------- ------------ -----------------
VERS       01-SEP-15        1.198             1.554
VERS       01-OCT-15        1.209             1.652
VERS       01-NOV-15          1.3             1.805
...
VERS       01-MAY-17        6.526             7.226
VERS       01-JUN-17        7.085             8.528
VERS       01-JUL-17        7.136             7.569

23 rows selected.

SQL>

The unit is in Tb, which should be suitable for most, however this can be changed by add/removing division of 1024.

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Thanks

Zed DBA (Zahid Anwar)