On occasion, the last journal section may be partially written in the case of a crash or a filesystem snapshot backup. This condition is expected and should be tolerated during recovery on a subsequent mongod startup, as the partially written last journal section can be safely ignored. However when this situation occurs it is not ignored, causing recovery to fail with the following log entries:
Assertion: 15874:couldn't uncompress journal section
When journal recovery encounters this situation, mongod refuses to start.
Affected users can run a 2.6.5 or 2.4.12 mongod to recover from this situation. This issue affects recovery only – database files after crashes and filesystem snapshot backups made under prior versons of mongod are healthy and uncorrupted, and are trivially recoverable by 2.6.5 and 2.4.12 mongod.
MongoDB production releases up to 2.6.4 and 2.4.11 are affected by this issue.
The fix is included in the 2.6.5 and 2.4.12 production releases.
Do not treat an incomplete last section of the journal as an error.