site_archiver@lists.apple.com Delivered-To: darwin-dev@lists.apple.com On Nov 18, 2004, at 6:28 PM, Igor Shmukler wrote: Was support for VFS stacking removed? I was warned of such a year or two ago, and stopped my VFS efforts at that point (also trying to intercept file system calls). Stacking filesystems in the BSD model doesn't work very well. It gets expensive very quickly, and locking can become a nightmare. It's also often not really what you want; the above is a good example of that. "also trying to intercept file system calls" Opinions on that differ to a considerable degree. = Mike _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-dev mailing list (Darwin-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-dev/site_archiver%40lists.appl... 1. Try VFS Stacking ? Or over-riding V-node ops ? The above being - ? SunOS is a proof that BSD model and stackable VFS do work well together. The reason locking in an operating system such as a BSD would problematic is because VFS is very complex. It needed flattening back in 1998 right after VM became stable. Now to support MP kernel locking is more complex than ever. That does not mean that things ought to be this way. Implementing a stackable FS is relatively straightforward if you start from the right place - vnode interface. If done correctly stackable does not create any problems with locking that would not be present in a leaf design. These are interesting assertions, but without any real application in the current context. I would caution anyone else reading this that your opinions are certainly not widely held. In particular, fan-in, fan-out and partial unwinds are all necessary for stacking filesystems (if you don't want unwinds, you need reservations which are arguably worse). Locking these is painful at best, and nesting vnodes is never not expensive. This email sent to site_archiver@lists.apple.com