Replication Oplog Window alerts and Oplog GB/Hour monitoring
Parent: Oplog sizing under storage auto-scaling · Published reference · snapshot 2026-10-31
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Definitions
- The retained oplog window is a time span; Percona defines its recovery-window panel as the difference between the newest and oldest oplog entries. Higher generation relative to configured oplog capacity shortens that window. (high confidence) [source]
How it works
- Percona's PMM oplog-sizing advisor warns when the window is below 24 hours. This is that advisor's policy; a minimum-window alert should be interpreted in the context of the workload and its recovery requirements. (medium confidence) (verified as of 2026-10-30) [source]
- Atlas aggregates historical metrics using averages of finer-grained reports. Consequently, a coarse historical view can smooth short generation bursts; examine available finer-grained data when sizing for peaks. (low confidence) [source]
- MongoDB recommends considering peak Oplog GB/Hour when increasing configured oplog size and notes that additional cluster storage may be needed to make room for the resize. Operators should therefore check storage availability alongside oplog capacity when resolving these alerts. (low confidence) [source]
Problems, failure modes and limitations
- A rate multiplied by a required recovery interval gives a starting capacity estimate. Debezium explicitly recommends anticipated GB/Hour multiplied by the time needed to address a connector failure; its example is 3 GB/hour times 2 hours equals 6 GB. (low confidence) [source]
- A sizing estimate is hourly oplog generation multiplied by the required number of recovery hours. Percona gives 24-hour and 48-hour examples; Debezium independently ties the interval to failure-response time. This is workload-dependent guidance, not a guaranteed future retention duration. (medium confidence) [source]
- Debezium recommends monitoring both Oplog GB/Hour and Replication Oplog Window to prevent failures from missing oplog entries; it warns that downtime exceeding the retained window, combined with generation faster than consumption, can cause connector failure. (low confidence) (verified as of 2026-10-27) [source]
Comparisons and alternatives
- MongoDB's cached Atlas architecture guidance recommends an oplog-window alert below 24 hours for 5 minutes at low priority, and below 1 hour for 10 minutes at high priority. It calls these workload-adjustable baseline recommendations, rather than universal thresholds. (low confidence) (verified as of 2026-10-14) [source]
- Percona PMM 3.6.0 release notes record PMM-14749: a missing query expression made the MongoDB Sharded Cluster Summary Oplog GB/Hour panel show No Data. A missing graph can therefore reflect a dashboard defect rather than zero oplog generation. (low confidence) (verified as of 2026-02-05) [source]
Facts and statements
- Oplog GB/Hour reports average gigabytes of oplog data generated by the primary per hour. MongoDB and Percona both recommend using it to assess oplog sizing. (high confidence) [source]
- Atlas documents Replication Oplog Window is (X) as a primary-oplog time-window alert at or below the configured threshold, and Oplog Data Per Hour is (X) as an hourly oplog-generation alert at or above its threshold. These measure time and data rate respectively. (high confidence) (verified as of 2026-10-14) [source]
- Write bursts can raise oplog generation and shorten the retained window. A rising GB/Hour graph together with a shrinking window is evidence to investigate oplog capacity relative to workload. (high confidence) [source]
- Monitor Oplog GB/Hour and Replication Oplog Window together. MongoDB recommends these alongside Replication Lag during lag investigations; Debezium independently recommends the first two to protect retained change history. (high confidence) [source]
- Increasing oplog capacity may also require increasing cluster storage to provide enough free space. Treat the observed retained window as a validation signal after a capacity change. (medium confidence) [source]
- Historical MongoDB issue SERVER-22717 reported a huge GB/hour spike without an Opcounters spike on version 3.0.9 and was closed as a duplicate of SERVER-22634, an oplog-delete data-size integer-overflow bug. This is a version-specific diagnostic lead, not evidence that modern spikes are generally false. (low confidence) [source]
- Percona PMM documents an advisor that warns below a 24-hour oplog window and generally recommends 24–48 hours. This product recommendation still requires matching the window to the deployment recovery requirement. (low confidence) (verified as of 2026-10-29) [source]
Children
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