Here is a related question that has been on my mind for a while, but I have yet to find a good answer for: If I write to file on a reasonably recent Linux and

Here is a related question that has been on my mind for a while, but I have yet to find a good answer for: If I write to file on a reasonably recent Linux and a sane file system like ext or zfs, at which point do I have the guarantee that when I read the same file back, that it is consistent and complete? Do I really need to fsync or is Linux smart enough to give me back the buffer cache? Does it make a difference if reader and and writer are in the same thread or same process?