# Time series bucketing parameters: granularity, bucketMaxSpanSeconds, bucketRoundingSeconds > Source-anchored research for Time series bucketing parameters: granularity, bucketMaxSpanSeconds, bucketRoundingSeconds. Run status: COMPLETED. Parent: [Bucket catalog memory and closure reasons](https://llms-explorer.com/tree/bucket-catalog-memory-and-closure-reasons/) · 5 facets · 8 facts · page: https://llms-explorer.com/tree/time-series-bucketing-parameters-granularity-bucketmaxspanseconds-bucketroundingseconds/ ## How it works - In MongoDB 7.1 and later, fixed-bucket query optimizations require a collection created with both custom parameters and no later parameter changes. For eligible predicates using $gte and $lt on the timeField, MongoDB can omit the per-measurement event filter when the query bounds align with bucket boundaries. (medium confidence) (verified as of 2026-10-22) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/query/timeseries/README.md) ## Measurements and reference values - The time-span setting is only one bucket constraint. MongoDB documents a maximum of 1,000 measurements or 125 KB per bucket, whichever is lower, so a configured maximum time span does not guarantee that a bucket remains open for that entire interval. (high confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/core/timeseries/timeseries-limitations.md) ## Problems, failure modes and limitations - During insertion, the BucketCatalog closes an open bucket if the incoming measurement would make the time between the bucket's oldest and newest timestamps exceed the collection's allowed span. A bucket closed for this reason remains eligible for reopening. bucketRoundingSeconds positions a new bucket's lower time boundary; it is not the span limit. (high confidence) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/timeseries/README.md) - After collection creation, collMod can increase granularity or the custom span and rounding values but cannot decrease them. Existing bucket documents are not rewritten when the setting changes; the new span applies to new buckets. (high confidence) (verified as of 2026-10-22) — [source](https://www.mongodb.com/docs/manual/core/timeseries/timeseries-limitations.md) - An October 2023 forum report describes MongoDB 7.0.2 rejecting a 300-second custom pair after an upgrade from 6.0.3 with the error that bucketMaxSpanSeconds was not configurable away from the default 3,600 for the provided granularity. A MongoDB respondent attributed the failure to an incomplete upgrade and advised following the release procedure; this is version-specific troubleshooting evidence. (low confidence) (verified as of 2023-10-09) — [source](https://www.mongodb.com/community/forums/t/problem-setting-bucketmaxspanseconds-and-bucketroundingseconds-in-mongodb-timeseries-collection/248318) ## Comparisons and alternatives - MongoDB 6.3 and later supports manual fixed-window bucketing with bucketMaxSpanSeconds and bucketRoundingSeconds instead of granularity. The two values must be equal, between 1 and 31,536,000 seconds inclusive, and granularity must be omitted. bucketMaxSpanSeconds limits the oldest-to-newest time range in a bucket. When a new bucket is opened, bucketRoundingSeconds rounds the first measurement's timestamp down to determine the bucket's minimum time. (high confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/core/timeseries/timeseries-granularity.md) ## Facts and statements - MongoDB describes granularity as the expected cadence between measurements. Its presets map to bucketRoundingSeconds and bucketMaxSpanSeconds as follows: seconds = 60 and 3,600; minutes = 3,600 and 86,400; hours = 86,400 and 2,592,000. (high confidence) — [source](https://www.mongodb.com/docs/manual/core/timeseries/timeseries-granularity.md) - In a September 2024 migration discussion, one user reported that custom 3,600-second bucketing made queries slower and the collection larger than using granularity: hours. This is a workload-specific report, not a general benchmark or a contradiction of the fixed-boundary rules. (low confidence) (verified as of 2024-09-04) — [source](https://www.mongodb.com/community/forums/t/migration-to-timeseries-collection/295416)