On Tue, Feb 17, 2026 at 3:54 AM Alyssa M via 9fans <[email protected]> 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.
------------------------------------------
9fans: 9fans
Permalink:
https://9fans.topicbox.com/groups/9fans/Te8d7c6e48b5c075b-M270677fd2081a0f5169e9a9e
Delivery options: https://9fans.topicbox.com/groups/9fans/subscription
