Thank you very much Marek and Quinn for your answers! I have started to take a look at webdavfs, it should be a good reference for me when trying to understand what the problems with Arla and OpenAFS are. Quinn wrote: ... it's not possible to create a stacking VFS plug-in that: 1. works reliably, Darwin inherited a VFS design from 4.4BSD that included stacking. However, Apple made significant changes to the VFS architecture (specifically, UBC and volfs) that affect the viability of stacking. I believe that these changes break stacking in the general case, Do you mean that the UBC design in general, on most OSes, break stacking, or is there something specific in the darwin implementation that does that? volfs I haven't even thought about, and neither do any of those two file system clients. I hope that we can instead rely on the emulation layer in the libraries for years to come. and no one in Apple file system engineering has the resources to create an example stacked file system that proves me wrong. Except for the (semi-stacking) webdavfs then, right? I don't mean to split hairs in a moot discussion, I just want to understand both the issues and the terminology. Regardless of the in-kernel problems there are also serious management problems for stacked file systems. I understand that those issues are important to apple and that it is hard to find a good general solution, but in this case I have the privilige to be able to ignore them! :-) However, we chose to support leaf file systems for the following reasons. ... o Leaf file systems are more important to our market than stacked file systems. As a member of the university computing community, I have an itching feeling that I must take that hat on for a moment and say that many of us universites are using AFS, and I hope we are important to apple. We are not alone, there are a few other, some very large, organizations that use it, but I must admit that AFS isn't very common in the general marketplace (yet! :-). For you engineers on this list, this is just information that I am happy if you have heard, I know we must use other channels to communicate our needs. o When we do have a kernel binary compatibility story (see below), it will likely address leaf file systems as a priority. I'd of course be very happy if you could consider those caching file systems too in your design. Should I contact someone for customer input, please let me know! It is good to know that apple wants webdav to work, the AFS clients could hopefully use the same mechanisms if there will be any. As a side discussion, I know that one of the Arla developers talked to Clark Warner about the possibility of generalising the webdav kext and point developers to that for both leaf and other file systems. That seems to be a good idea. (The kernel module that arla is using (formerly known as xfs, now called nnpfs), is actually meant to be such a general purpose cross platform kernel module, but maintaining it is a lot of work.) Since then we've been working hard to flesh out the KPI story for VFS plug-ins and NKEs. I'm hoping that we'll cover this in depth at WWDC 2004. So do I, see you there! Thanks again for your help! /ragge _______________________________________________ darwin-kernel mailing list | darwin-kernel@lists.apple.com Help/Unsubscribe/Archives: http://www.lists.apple.com/mailman/listinfo/darwin-kernel Do not post admin requests to the list. They will be ignored.