Answer
MongoDB truncates old oplog entries automatically. If the oplog is too small, secondaries cannot catch up, causing rollback or forcing an initial sync that increases downtime risk.
Quick Answer
The oplog is a capped collection - when it fills up, oldest entries are truncated. If a secondary falls behind more than the oplog window (oplog can hold N hours of operations), the secondary can't catch up via replication and needs a full resync (initial sync). Risk: a resync during heavy load is very slow. Size oplog to cover your longest expected maintenance window.
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SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ