This checks the fsync time, if the configured value (defaults to 1second) is exceeded a warning is printed to the log.
I copied this concept from HBase, except in this case it's override-able if necessary. (note docs change).
Nearly all existing tests verify this code path, I ran with a much lower configured value and verified this message is output to the log correctly when threshold exceeded.
There is a patch for 33 and one for 34/trunk.
I've marked this for 3.4.0 because I think it's both low risk and very useful to have (keep seeing this issue pop up in user reports, which will allow Filesystem diagnosis.