Under extremely rare circumstances, a race condition in the code that updates large records may cause some of those updates to be lost during an unclean shutdown.
On a production system, the path with the race condition is only taken when log records are 128k or larger. From MongoDB's perspective, it is a smaller record size, maybe 40k, since an individual WT log record contains the insert into collections, indexes, oplog, etc.
Attempts to trigger this race condtion with MongoDB using a synthetic workload with compression disabled have produced mixed results. However, attempts to reproduce this issue in MongoDB with default compression (snappy) have been unsuccessful.
This issue only affects users running with journaling enabled. Users that run with journaling disabled can not be affected by this bug.
If the race condition is triggered and the node suffers an unclean shutdown, some updates to large records since the last checkpoint may be lost. Unfortunately it is not possible to detect if the race condition has been triggered.
MongoDB 3.2 versions up to and including MongoDB 3.2.7.
A fix for this issue is included in the MongoDB 3.2.8 production release. Users with workloads that include updates to large records whose nodes may be subject to unclean shutdowns should upgrade to MongoDB 3.2.8 to avoid exposure to this issue.
Unfortunately there are no known workarounds for this issue.