Behaviour of --append with modified files

[Mark Stead]

We have a rsync'd filesystem which generally contains very large files that only 
grow.
We've specifically used the --append option to handle these files - since 
otherwise rsync is effectively CPU bound the whole time.

I'd always assumed that any files that were simply modified (without a change in 
length) would be handled under the existing rsync logic.  However it appears 
that once --append is selected then existing files are never copied/updated 
based on timestamp, only when the length changes.

Is there any option that I've overlooked whereby I can force rsync to 
synchronise modified files (in addition to handling appended files).

I could do two passes (--append followed by normal non-append), but the second 
pass is likely to also synchronise files that are appended to and rsync will end 
up taking 8 hours to complete.
Maybe my best option is to use two rsync passes, and add use the 
--max-size/--min-size to ensure that append is only applied to large files, and 
to apply standard rsync logic for smaller files.  This assumes that I can 
guarantee that files beyond a specific size will only be appended to.

Thanks for any assistance.
Mark