sync_file_range(2) allows you to specify ranges to sync, instead of the whole file, but it's not that useful anyway.

In most DBs, you have separate data and log. The log is always only appended to, and the nature of disks is such that grouping concurrent transactions together into a single commit is a big win regardless of how syncing works. So syncing all outstanding I/O to the log file is generally what you want to do anyway.

While the data store is always being written to, its writes do not need to be synced to disk until a log checkpoint is made (and the old log subsequently discarded). And since that is an intermittent asynchronous process, there is not much to gain from finer-grained syncing here either.

See e.g. http://www.postgresql.org/message-id/4A51CB76.5020407@enterp...