When diagnosing issues related to i/o capacity it may be necessary to account for all i/o generated by mongod. Currently we monitor in FTDC disk usage for WT data files and WT journal, and can easily compute mongod log usage.

However we don't record i/o volume (bytes written and read) for temporary files used by queries for sorting, index builds, etc. Can we record this in FTDC? In order to be useful this would need to be recorded as a running total as the query proceeds (as opposed to simply bumping a counter at the end of the operation).