Re: Memory mapped files question

[Jamie Lokier <jamie@shareable.org>]

H. Peter Anvin wrote:
> > Hi, everyone.  Thanks for all your responses.  Our confusion is
> > that in Unix environments, when we modify memory in memory-mapped
> > files the underlying system flusher manages to flush the files for
> > us before the files are munmap'ed or msysnc'ed.
> 
> Bullshit.  It might work on one particular Unix implementation, but
> the definition of Unix, the Single Unix Standard, does explicitly
> *not* require this behavior.

I presume that if you do write(), the Single Unix Standard allows the
data to remain dirty in RAM for an arbitrary duration too.

If I write() a file I expect it to be automatically written to disk
within a few minutes at most, where that is plausible.

Frank van Maarseveen wrote:
> Shared mmaped files are _never_ flushed, at least in 2.4.x. So,
> without an explicit msync() a process (innd comes to mind) may loose
> years of updates upon a system crash or power outage.

It's a quality of implementation issue if data can remain dirty in RAM
forever without ever being flushed.

Can this really happen with normal open/mmap/munmap/close usage, or
does it only occur with long-lived processes like innd which mmap a
file, dirty the pages but never munmap them?

If the former case does happen, I'd say we're failing on quality of
implementation.  If it's only the latter case, though, fair enough: the
application writer will have to use msync().

-- Jamie