[jira] [Created] (HDDS-16753) NumSnapshotDeleted is not decremented when a snapshot is purged

["Chi-Hsuan Huang (Jira)" <ji...@apache.org>]

Chi-Hsuan Huang created HDDS-16753:
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             Summary: NumSnapshotDeleted is not decremented when a snapshot is purged
                 Key: HDDS-16753
                 URL: https://issues.apache.org/jira/browse/HDDS-16753
             Project: Apache Ozone
          Issue Type: Bug
          Components: Ozone Manager
            Reporter: Chi-Hsuan Huang
            Assignee: Chi-Hsuan Huang


Follow\-up to HDDS\-16719.

{{OMMetrics.numSnapshotDeleted}} is incremented in {{OMSnapshotDeleteRequest}} when a snapshot is marked {{SNAPSHOT\_DELETED}}, but nothing decrements it when {{SnapshotDeletingService}} purges that snapshot:

* {{OMSnapshotPurgeRequest}} removes the entry from {{snapshotInfoTable}} and only calls {{omSnapshotIntMetrics.incNumSnapshotPurges\(\)}}.
* {{decNumSnapshotDeleted\(\)}} has had no callers since it was added in HDDS\-7506.

As a result, {{NumSnapshotDeleted}} keeps growing on a long\-running OM and reflects "snapshots deleted since startup" rather than "snapshots waiting to be purged". The value is only corrected when {{updateActiveSnapshotMetrics\(\)}} recounts the table on OM start or checkpoint reload. The bundled OM Snapshot dashboard plots this raw value, so the drift is visible there.

Proposed changes:

* Call {{decNumSnapshotDeleted\(\)}} in {{OMSnapshotPurgeRequest}} for each snapshot actually purged, skipping the case where the snapshot is already gone from the table.
* Assert {{NumSnapshotDeleted}} in {{TestOMSnapshotPurgeRequestAndResponse}}.





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