On SmartOS msync is implemented in a very inefficient manner - it has to scan every page of the mapped region to determine which files to sync. This uses a lot of CPU time for large dbs and
- on 2.4 can cause pauses in the app due to a lock held while doing the background flush
- on 2.6 can cause pauses in the app during background flush due to CPU contention
On the other hand fsync of the underlying file descriptor is much more efficient, doing only the work necessary to flush the dirty pages.