Collection statistics are exposed by "db.stats()" and  "count()" commands. They are stored in 2 different WT tables:

• sizeStorer.wt contains dataSize, objects (aka numRecords, used by "count()" command)
• WiredTiger.wt contains storageSize, freeStorageSize, indexSize, indexFreeStorageSize, etc.

Both kinds of information are copied from D to R via a temporary WiredTiger instance. The sizeStorer.wt table is synced from in-memory stats every 60 seconds, so the copied info will probably be stale, leading to inaccurate fast count forever.

PM-2947 will be redesigning the fast count tracking mechanism and ensure eventual fast count consistency. After PM-2947 completion, we will use this ticket to investigate if Merge can ensure eventually consistent fast count with reasonable effort.