During the port of MMP (illumos bug 10499) from ZoL, I found this
earlier ZoL project is a prerequisite. Here is the original
description.  This addition will enable us to sync an open TXG to the
main pool on demand. The functionality is similar to 'sync(2)' but
'zpool sync' will return when data has hit the main storage instead of
potentially just the ZIL as is the case with the 'sync(2)' cmd.