How tablespace usage is calculated
How the Oracle Management Pack calculates tablespace and temp tablespace usage, and why the result can differ from your own query.
Applies to: Opslogix Oracle Management Pack (all versions)
Last updated: October 2026 · Latest version: 26.4.4517.0
See also: Upgrade guide for 26.4.4517.0 · Version support · Release notes
The "Table Space Used %" monitor and the tablespace performance rules don't use Oracle's DBA_TABLESPACE_USAGE_METRICS view. They calculate usage themselves, so the result can differ from the query your DBA uses. This article explains the calculation.
Permanent tablespaces
- Maximum size: the sum over all data files of the tablespace. For a data file with autoextend on, its maximum size (MAXBYTES) is used; for other data files, their current size.
- Used: the allocated size minus the free space in DBA_FREE_SPACE. For the SYSTEM tablespace, the sum of all segments in DBA_SEGMENTS is used instead.
- Used % = Used ÷ Maximum size × 100. Free (MB) = Maximum size − Used.
Because autoextend is taken into account, a tablespace that has used most of its current size but can still grow is not reported as full. A query that only looks at the allocated size shows a higher percentage.
Temporary tablespaces
- Maximum size: the sum of the temp files, using the maximum size for temp files with autoextend on.
- Used: the used blocks in V$SORT_SEGMENT multiplied by the database block size.
- Used % = Used ÷ Maximum size × 100.
Default thresholds
| Monitor | Warning | Critical | Enabled |
|---|---|---|---|
| Table Space Used % | above 85 % | at or above 95 % | yes |
| Temp Table Space Usage Pct | above 85 % | at or above 95 % | yes |
| Table Space Free MBs | below 50 MB | at or below 5 MB | no |
| Table Space Used MBs | above 1000 MB | at or above 2000 MB | no |
All monitors run every 5 minutes and only on primary databases. You can change the thresholds per tablespace with overrides; see How to change thresholds and intervals with overrides. For very large tablespaces an absolute threshold in MB is often more useful than a percentage: enable "Table Space Free MBs" with an override.
Usage above 100 %
Older versions could report more than 100 % for some tablespaces. Upgrade to the latest version of the Oracle Management Pack.