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 <[email protected]> wrote: > 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-Med980740c04af0912ebaf0f0 Delivery options: https://9fans.topicbox.com/groups/9fans/subscription
