25 TB seems quite a lot.
Are those like billions of small files perhaps ?
Is content is constantly adding and to what extent ?
I have no experience lsyncd with but from Lsyncd github page :
Quote:
Lsyncd is designed to synchronize a local directory tree with low profile of expected changes to a remote mirror.
This sentence would suggest that mentioned software does 'not like' source changing a lot, someone please correct me
As for inotify, read the 'Limitations and caveats' part of
inotify(7) - Linux manual page
There are some stuff there which might interest you if you have large number of watchers.
Hope that helps
Regards
Peasant.