Re: mmaping on plan9? (was Re: [9fans] venti /plan9port mmapped

[ron minnich <rminnich@gmail.com>]

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I suggest we start over, and, as Dan says, let's stop using the word mmap.

Can we look at this from the point of view of a problem you are trying to
solve? What is it you want to do?

On Tue, Feb 17, 2026 at 5:36 AM Dan Cross <crossd@gmail.com> wrote:

> On Tue, Feb 17, 2026 at 3:54 AM Alyssa M via 9fans <9fans@9fans.net>
> wrote:
> > Guys, I'm really sorry for the confusion.
> > A week ago I posted this, as a loose analogy, trying to relate what Ron
> said to my earlier idea:
> > [snip]
> > > It would be convenient for this if servers for disk file systems had
> the ability to create a snapshot of a range of bytes from a file. But they
> generally don't. So I'm building a file system wrapper layer (a file server
> that talks to another file server) that provides snapshot files to the
> kernel via an extension of the file server protocol, in addition to what
> the underlying file system already provides. My current implementation does
> this with temporary files. When a file is written, the temporary file gets
> any original data that's about to be overwritten. The snapshot provided to
> the kernel is a synthetic blend of the original file and any bytes that
> were rescued and put in the temporary file. In most uses the original file
> will never be touched by another process and the temporary file won't even
> be created.
> > >
> > > The wrapper requires exclusive access to the file server underneath,
> and also requires the contents to be stable. It is the wrapper that is
> mounted in the namespace. So the wrapper sees all attempts to alter any
> file, and can ensure that that any snapshot maintains the illusion of being
> a full prior copy when writes later happen to the file it came from.
>
> This doesn't seem workable.
>
> Consider a network with three machines: one serves a filesystem, two
> mount it as clients. If I understand your description above, there is
> one of these "wrappers" on each client. How, then, do they arrange for
> exclusive access to the filesystem, which is on a completely separate
> machine?
>
> > [snip]
> > I'm dismayed by the responses so far, because I think this is
> potentially a lot better than mmap.
> 
> No, this is nothing like mmap.  mmap is an operation initiated by a
> programmer; this is some totally separate thing.
> 
> My humble suggestion is that if you don't want people to conflate this
> with the `mmap` call, you should stop referring to it as `mmap`.
> 
> - Dan C.

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