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.
{
    if (fd < 0)
    {
        assert(!offset() && "attempt to write after close");
        return;
    }
    
    MemoryTracker::LockExceptionInThread lock(VariableContext::Global);
    next();
}
void WriteBufferFromFileDescriptor::sync()
{
    
    next();
    
    int res = fsync(fd);
    if (-1 == res)
        throwFromErrnoWithPath("Cannot fsync " + getFileName(), getFileName(), ErrorCodes::CANNOT_FSYNC);
}
off_t WriteBufferFromFileDescriptor::seek(off_t offset, int whence)