I am new to Oracle, and I am not quite understanding the size calculations shown in OEM under tablespaces. I know it should be simple and my well be.

I need to monitor the table space but not sure what I am looking at. Should I go after the "Available Space Used(%) or the Allocated Space Used(%) to monitor?

If I am running a script to alert when a threshold is met, which % should I use?

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1 Answer 1


"Allocated Space Used(%)" reports the utilization of a tablespace based on the current size of its datafiles. Typically the value of this metric is close to 100% which causes a lot of false positive alerts.

Go after "Available Space Used(%)". That reports the utilization of a tablespace based on the configured maximum size of its datafiles (and the available free space in the filesystem or ASM diskgroup).


Tablespace TS1 consist of a single datafile that is 10 GB and contains 10 GB data. The datafile is configured as autoextensible with a maximum size of 20 GB. The single datafile of TS1 is on a filesystem with more than 10 GB free space.

"Allocated Space Used(%)" - 100%
"Available Space Used(%)" - 50%

If the single datafile of TS1 is in a filesystem with only 5 GB free space:

"Allocated Space Used(%)" - 100%
"Available Space Used(%)" - 66.6%
  • Thank you Balazs.. That is exactly what I needed. This looks to be safe just using a "select tablespace_name, used_percent from dba_tablespace_usage_metrics". That percent used lines up with Available Space Used in OEM. Thank you again!! Dec 28, 2023 at 23:47

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