For WiredTiger, a background thread manages the size of the oplog, truncating it when necessary. However the piece that waits to truncate the oplog is holding onto a borrowed OperationContext. Waiting for a truncation point can be arbitrarily long, particularly when there are few writes coming into the system, or when the oplog has yet to start "rolling over".
- related to
SERVER-31101 WT table not dropped after collection is dropped due to long-running OperationContext