Looking at these two lines of source in WT and MMapV1:

WT: https://github.com/mongodb/mongo/blob/r2.8.0-rc4/src/mongo/db/storage/wiredtiger/wiredtiger_record_store.cpp#L353
MMapV1: https://github.com/mongodb/mongo/blob/r2.8.0-rc4/src/mongo/db/storage/mmap_v1/record_store_v1_capped.cpp#L403

Note that in WT a '>' is used and in MMapV1 a '>=' is used. Given a capped collection of size 100, if we insert two documents of size 50 and then truncation occurs:

• in MMapV1, one document will get deleted because its position would be >= 100
• in WT no documents would be deleted because the size would not be > 100.

These two behaviors should be made the same, likely in favor of the existing MMapV1 behavior.