# Index build commit quorum and hung builds > Source-anchored research for Index build commit quorum and hung builds. Run status: COMPLETED. Parent: [Index build resource limits (maxNumActiveUserIndexBuilds, disk space)](https://llms-explorer.com/tree/index-build-resource-limits-maxnumactiveuserindexbuilds-disk-space/) · 5 facets · 19 facts · page: https://llms-explorer.com/tree/index-build-commit-quorum-and-hung-builds/ ## Definitions - For simultaneous replicated builds, votingMembers is the default commit quorum and requires all data-bearing voting members, including the primary, to become ready before commit. (high confidence) — [source](https://www.mongodb.com/docs/manual/reference/command/createIndexes.md) ## How it works - SERVER-71768 reports Atlas builds stalling when a secondary already has the index and skips voting, with interrupted rolling builds suspected. Its documented workaround is adjusting the quorum. (medium confidence) — [source](https://jira.mongodb.org/browse/SERVER-71768) ## Problems, failure modes and limitations - setIndexCommitQuorum has no effect on a build started with commitQuorum: 0. Members configured with buildIndexes: false cannot participate in a commit quorum. (low confidence) — [source](https://www.mongodb.com/docs/manual/reference/command/setIndexCommitQuorum.md) - SERVER-56976 records a historical bug where changing quorum failed to wake a build already waiting for votes. The issue lists fixes in 4.4.7, 5.0.0-rc0, and 5.1.0-rc0; this does not establish failure in current versions. (low confidence) — [source](https://jira.mongodb.org/browse/SERVER-56976) - setIndexCommitQuorum cannot alter a build started with commitQuorum: 0. (low confidence) — [source](https://www.mongodb.com/docs/manual/reference/command/setIndexCommitQuorum.md) - A member configured with buildIndexes: false cannot participate in commit quorum; a voting member with this setting prevents use of votingMembers. (high confidence) — [source](https://www.mongodb.com/docs/manual/reference/command/createIndexes.md) - Starting in MongoDB 7.1, low disk space can trigger index-build abortion, but a member that has already voted to commit cannot retract its vote to request abortion. Do not assume the disk-space threshold always stops a waiting build. (high confidence) (verified as of 2026-10-14) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/index_builds/README.md) ## Comparisons and alternatives - Percona reproduced a default-quorum build waiting with one of three data-bearing members down; currentOp showed "Index Build: draining writes received during build". This message can therefore reflect quorum waiting rather than ongoing application writes. (medium confidence) — [source](https://www.percona.com/blog/commitquorum-in-index-creation-from-percona-server-for-mongodb-4-4/) ## Facts and statements - The default commitQuorum is "votingMembers": all data-bearing voting replica-set members. An unreachable required member can leave an index build hanging until it returns. (medium confidence) — [source](https://www.mongodb.com/docs/manual/reference/command/createIndexes.md) - Members vote after scanning the collection and the first side-write drain. While awaiting the commit decision, the build continues applying side writes; finishing the scan is not equivalent to completing the build. (high confidence) — [source](https://github.com/mongodb/mongo/blob/master/src/mongo/db/index_builds/README.md) - setIndexCommitQuorum changes an in-progress build’s quorum. indexNames must include the entire group of indexes created by its single createIndexes operation. (high confidence) — [source](https://www.mongodb.com/docs/manual/reference/command/setIndexCommitQuorum.md) - Lowering commit quorum can let the primary commit sooner, but an unfinished secondary receiving commitIndexBuild can block further oplog application until its local build finishes. (high confidence) — [source](https://www.mongodb.com/docs/manual/reference/command/setIndexCommitQuorum.md) - Commit quorum controls readiness before primary commit. Starting in MongoDB 8.0, write concern waits for replication of the index-build oplog entry; earlier releases waited for the specified nodes to finish building the index. (high confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/reference/write-concern.md) - SERVER-87929 reports that changing a member's votes to zero did not update the quorum of an already-running build until primary stepdown or explicit quorum adjustment. The issue was resolved Won't Do in April 2024; treat this as a documented case, not a universal current-version guarantee. (low confidence) — [source](https://jira.mongodb.org/browse/SERVER-87929) - In a 2020 firsthand report, a four-document collection hung with the draining-writes message despite no application writes. The author reported a node self-communication problem and a keyfile-related resolution after consulting MongoDB; this is historical case evidence, not a universal workaround. (low confidence) — [source](https://stackoverflow.com/questions/63670703/mongodb-stuck-creating-index-with-index-build-draining-writes-received-during) - SERVER-115085 describes an initial-sync member delaying index-build quorum until collection cloning finishes and index-build oplog entries are applied. The proposed automatic adjustment was closed Works as Designed, so the proposal is not evidence that automatic quorum reduction shipped. (low confidence) — [source](https://jira.mongodb.org/browse/SERVER-115085) - The manual directs operators to dropIndexes or its shell helpers to stop in-progress replicated builds, and warns against using killOp for replica sets or sharded clusters. (low confidence) — [source](https://www.mongodb.com/docs/manual/core/index-creation.md) - An unreachable data-bearing voter can hold a default-quorum index build indefinitely; Percona reproduced a build completing after the missing third member returned. (high confidence) — [source](https://www.mongodb.com/docs/manual/core/index-creation.md) - Inference: quorum waiting can prolong the lifetime of active builds, so increasing maxNumActiveUserIndexBuilds does not repair an unsatisfied quorum and can admit additional resource-consuming work. (medium confidence) — [source](https://www.mongodb.com/docs/manual/core/index-creation.md)