hard links to files on other filesystems disallowed: why?
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From: The Natural Philosopher on 20 Feb 2010 17:33 Rahul wrote: > The Natural Philosopher <tnp(a)invalid.invalid> wrote in news:hlpfid$b1j$3 > @news.albasani.net: > >> There is a structural reason. >> >> Hard links are duplicate directory entries actually on the disk. >> >> They cannot refer to another disk. >> > > Thanks for the explanations guys. I think I understand how it is done. Buy > nothing in these explainations precludes why the links cannot point to a > inode on another filesystem that is also mounted. If two filessytems are > both mounted all storage on them is pretty much identical in terms of > inodes etc. , right? Or maybe I don't see the obvious.... > because the directory entry has no concept of other disks, or even partitions. It is just a number..the inode number.
From: Robert Riches on 20 Feb 2010 18:47 On 2010-02-20, Rahul <nospam(a)invalid.invalid> wrote: > The Natural Philosopher <tnp(a)invalid.invalid> wrote in news:hlpfid$b1j$3 > @news.albasani.net: > >> There is a structural reason. >> >> Hard links are duplicate directory entries actually on the disk. >> >> They cannot refer to another disk. >> > > Thanks for the explanations guys. I think I understand how it is done. Buy > nothing in these explainations precludes why the links cannot point to a > inode on another filesystem that is also mounted. If two filessytems are > both mounted all storage on them is pretty much identical in terms of > inodes etc. , right? Or maybe I don't see the obvious.... If you were to attempt that and were able to find some way to refer to the other filesystem, what you're proposing would _BE_ a symbolic/soft link. -- Robert Riches spamtrap42(a)verizon.net (Yes, that is one of my email addresses.)
From: John Hasler on 20 Feb 2010 19:12 Robert Riches writes: > If you were to attempt that and were able to find some way to > refer to the other filesystem, what you're proposing would _BE_ a > symbolic/soft link. No it wouldn't. A symlink is a file that contains a path in text form. You could extend directory entries to include a filesystem identifier (such as a UUID) as well as an inode but you'd have to standardize that identifier over all the kinds of filesystems Linux can mount. Not likely that Apple and Microsoft would cooperate. Besides, we already have symlinks. -- John Hasler jhasler(a)newsguy.com Dancing Horse Hill Elmwood, WI USA
From: Rahul on 20 Feb 2010 19:31 John Hasler <jhasler(a)newsguy.com> wrote in ews:87r5ofqwf8.fsf(a)thumper.dhh.gt.org: > likely that Apple and Microsoft would cooperate. Besides, we already > have symlinks. > Subject: Re: hard links to files on other filesystems disallowed: why? From: Robert Riches <spamtrap42(a)verizon.net> > If you were to attempt that and were able to find some way to > refer to the other filesystem, what you're proposing would _BE_ a > symbolic/soft link. Robert and John's response leads to this question: "Are there things that one can do with hardlinks which one cannot do with symlinks? If I somehow enabled cross-filesystem hardlinks is that the same as current soft-links? I think not." But if hardlinks have advantages that soft links don't then at least I have a case for "somehow" wanting cross-filesystem hardlinks. Of course, this might be stupid or impossible for other reasons. One example that I can think of where hardlinks are essential is: I've been doing rsnapshot / rsync differential backups that implicitly rely on hardlinks unique peoperties. Can't do that with softlinks. Maybe there are other situations where hardlink's alone will do? -- Rahul
From: Rahul on 20 Feb 2010 19:32
Jerry Peters <jerry(a)example.invalid> wrote in news:hlpmu1$331$5 @news.eternal-september.org: > Let's suppose it also contained some sort of FS info. What would that > be? It can't be a device number because there's no guarantee that the > device number is a constant (USB devices in particular). So how do you > "point" to the other FS? > How about a uuid as the identifier? Then if I remounted the same system so long as I use the same uuid the cross-filesystem hardlinks become consistant again. -- Rahul |