The comment on checkFile states this is “open a direct_io” to a new file. However, the O_DIRECT flag is not added to the open call. This should mean this is a standard, buffered IO.

https://github.com/mongodb/mongo/blob/master/src/mongo/watchdog/watchdog.cpp#L467

In an asymmetric storage failure where writes can complete, but reads are blocked, the read at https://github.com/mongodb/mongo/blob/master/src/mongo/watchdog/watchdog.cpp#L522 could be serviced from the Linux page cache, erroneously returning a success.