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https://issues.apache.org/jira/browse/HDDS-16694?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Dian-Xuan Yang reassigned HDDS-16694:
-------------------------------------
Assignee: Dian-Xuan Yang
> A failed snapshot load still increments NumSnapshotCacheSize
> ------------------------------------------------------------
>
> Key: HDDS-16694
> URL: https://issues.apache.org/jira/browse/HDDS-16694
> Project: Apache Ozone
> Issue Type: Bug
> Reporter: Siyao Meng
> Assignee: Dian-Xuan Yang
> Priority: Minor
> Attachments: HDDS-16694-CR-T1-cache-size-gauge-drift.patch
>
>
> h3. Mechanism
> {{omMetrics.incNumSnapshotCacheSize()}} sits inside the {{v == null}} branch
> of {{SnapshotCache.get}} ({{SnapshotCache.java:225-249}}) and runs even when
> the load threw {{OMException(FILE_NOT_FOUND)}} and no map entry was created.
> Both decrement sites require a mapped entry, so the gauge can only rise.
> h3. Impact
> The JMX {{NumSnapshotCacheSize}} gauge an operator uses to size the cache
> drifts upward monotonically on a routine condition, namely reading an already
> purged snapshot. Monitoring signal only, with no data or availability effect.
> h3. Reproduction
> {{repro/test_bugCR-T1_cache_size_gauge_drift.sh}}. Evidence in
> {{repro/logs/ev-cr.log}}.
> h3. Patch
> {{HDDS-XXXXX-CR-T1-cache-size-gauge-drift.patch}} is attached. It applies to
> 20e6a1c0f6039248373efd3eb1d7f2afcf7f1535 and carries the fix plus a
> regression test that was verified to fail without the fix and pass with it.
> {code:none}
> mvn test -pl hadoop-ozone/ozone-manager -Dtest=TestSnapshotCache
> {code}
> Found by code review of the OM snapshot cache under HDDS-15926, on commit
> 20e6a1c0f6039248373efd3eb1d7f2afcf7f1535. Generated with Specula (Claude Opus
> 5).
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