Summary

Created in relation to HELP-34247.

WiredTiger offers two methods for storing stats:
1. Write the stats directly to a WiredTigerStats file, or
2. Allow for stats to be queried by via an open session

MongoDB wants to store all stats in a single FTDC file, and to do so required querying WT stats as per 2. above. To save on wasted CPU they also disable writing to WiredTigerStats. However, WiredTiger requires an open connection to open a session and does not allow open sessions on shutdown which prevents MongoDB from accessing WT stats and saving them to FTDC files during open and shutdown. During these open/shutdown, WiredTiger could execute long running operations (e.g. RTS) where the stats are very useful.

To address this we should look into how WiredTiger can still provide these stats to MongoDB. For example investigating if it possible to:

• Have two step open/shutdown so that the long running aspects can have stats
• Have a separate stats cursor that could be access with less restrictions
• Allow read-only sessions on WiredTiger shutdown, letting MongoDB to continue querying stats
• Have WiredTiger save open/shudown stats to WiredTigerStats even when configured to report stats via sessions. Then allow Mongo to perform some post operation processing to aggregate these stats with the FTDC stats

Motivation

• Does this affect any team outside of WT?
  Yes, as other teams can use the now-available stats for analysing HELP/BF/Customer tickets.

• How likely is it that this use case or problem will occur?
  Infrequently on tickets related to WT open/shutdown (but frequent on huge datasets)

• If the problem does occur, what are the consequences and how severe are they?
  Not critical. This slows down analysis of HELP/BF tickets

• Is this issue urgent?
  No

Acceptance Criteria (Definition of Done)

A solution is identified for how to provide WT stats to MongoDB on WT open/shutdown.