Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uwm.edu!ux1.cso.uiuc.edu!ux1.cso.uiuc.edu!aglew From: aglew@dwarfs.csg.uiuc.edu (Andy Glew) Newsgroups: comp.arch Subject: Re: HP-PA and MPE XL page architecture Message-ID: Date: 28 Feb 90 03:42:54 GMT References: <37774@cornell.UUCP> <1990Feb27.024818.510@esegue.segue.boston.ma.us> Sender: news@ux1.cso.uiuc.edu (News) Distribution: comp Organization: University of Illinois, Computer Systems Group Lines: 15 In-Reply-To: johnl@esegue.segue.boston.ma.us's message of 27 Feb 90 02:48:18 GMT >We spent a long time arguing out what the correct semantics of copy-on-write >file mapping should be to support transactions, for example if two processes >map a file COW, should they get the same COW segment or separate ones. (The >answer, surprisingly, turns out to be that they get the same one.) The >semantics are non-trivial, to the extent that IBM patented the way it works, >no. 4,742,450. Copy-on-write filesystems are certainly useful. Dave Yost's "Rich Person's Configuration Control System" (USENIX Summer 1985) is basically COW on file granularity (I carried the same idea forward). Finer granularity would be nice. SCCS/RCS, if you look at them the right way, are COW files. All the arguments for file mapping apply to mapping of COW files. -- Andy Glew, aglew@uiuc.edu