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

[Dan Cross <crossd@gmail.com>]

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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