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.
int fd_,
    size_t buf_size,
    char * existing_memory,
    size_t alignment,
    std::string file_name_)
    : WriteBufferFromFileBase(buf_size, existing_memory, alignment)
    , fd(fd_)
    , file_name(std::move(file_name_))
{
}
WriteBufferFromFileDescriptor::~WriteBufferFromFileDescriptor()