Date: Thu, 26 Apr 2001 17:49:33 +0000 (GMT) From: Terry Lambert <tlambert@primenet.com> To: bright@wintelcom.net (Alfred Perlstein) Cc: mckusick@FreeBSD.ORG, fs@FreeBSD.ORG Subject: Re: versioned files via snapshot evilness. Message-ID: <200104261749.KAA24236@usr05.primenet.com> In-Reply-To: <20010426093439.J18676@fw.wintelcom.net> from "Alfred Perlstein" at Apr 26, 2001 09:34:39 AM
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> I'm wondering if it might be possible to abuse the ffs snapshot > stuff to do file versioning ala VMS. Not really, IMO. Versioning ala VMS requires kernel globbing, or amazingly intrusive library modifications, coupled with a guarantee by programmers that they will only use the preferred APIs in their applications. Consider how you would make: fd = open( "foo", O_RDWR, 0); open version 17 of the file "foo", as opposed to an earlier version, and how you would tag versions into the file names, without stealing a character (e.g. ";"). Also note that "/" is valid in a UTF-8 filename today, if it is part of a multibyte sequence (but FreeBSD can not handle this). The only permissable namespace escape mechanism on UNIX is the POSIX "//<namespace>/<path>" sequence. Unfortunately, FreeBSD's "namei", "lookup", and individual per-FS "VOP_LOOKUP" routines are broken, in that a namespace escape would need to be specified per-path component, since it could not be inherited by subsequent components. Fixing this is hard (but not impossible; I've done it twice now), and without the fix, you end up with "//<namespace>/<component>//<namespace>//<component>" to get a full path, or you get "/<component>//<namespace>/<component>" (both of these latter are illegal, since POSIX only permits the escape at the start of the path). You could create a "poor man's versioning system" using a modified union FS (to take the initial namespace escape) mounted over top of a folded namespace FS, mounted over e.g. FFS, to do the job of letting you say "//<version>/<path>" or just "<path>", if you only wanted the latest version. But this would still leave you with the problem of how to _list_ which versions were available for any given file, which -- again -- requires globbing in the kernel. Make sense? Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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