File size is updated correctly according to

fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled. For example, changes to st_atime or st_mtime (respec‐
tively, time of last access and time of last modification; see inode(7)) do not require flushing because they are not necessary for a subsequent data read to be handled correctly. On the other hand, a change to the file size
(st_size, as made by say ftruncate(2)), would require a metadata flush.

And we don't care about modification times.
Should work alright.