Implemented zpool sync command
zfs - Zettabyte File System
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.