Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!husc6!bu-cs!mit-eddie!genrad!decvax!decwrl!sun!gorodish!guy From: guy%gorodish@Sun.COM (Guy Harris) Newsgroups: comp.unix.questions Subject: Re: S5 file system magic number (was (Re: Pixel/80 trashed fs) Message-ID: <10848@sun.uucp> Date: Mon, 29-Dec-86 02:12:56 EST Article-I.D.: sun.10848 Posted: Mon Dec 29 02:12:56 1986 Date-Received: Mon, 29-Dec-86 18:36:58 EST References: <718@cooper.UUCP> <145@piaget.UUCP> <125@tdi2.UUCP> Sender: news@sun.uucp Lines: 15 Summary: Old UNIX file systems didn't have a magic number, so it's treated as optional. > ...but I noted later that sVr2 has a magic number in the superblock. > ...My question: if there is a magic number there, why doesn't mount() look > for it and reject file systems with bad magic numbers? Because the magic number wasn't in the original V7 file system, used until S5. S5 added a magic number to the V7 superblock, but only to distinguish 512-byte-block file systems from 1024-byte-block file systems. Since the original V7 file system used 512-byte blocks, any file system with an unknown magic number was treated as a 512-byte-block file system. Requiring a valid magic number would have required that the root file system be fixed before an S5 kernel was booted on it, unless the initial mount of the root file system was treated as a special case, and required that a file system be fixed before it was mounted by that kernel, unless a special option were added to the "mount" system call. I figure they just decided this was too much trouble.