Re: [TUHS] moving directories in svr2

[Rob Pike <robpike@gmail.com>]

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Not an answer, but relevant: Apple's Time Machine works by hard-linking
directories to make the forest of old roots.

-rob


On Thu, Dec 30, 2021 at 5:28 AM ron minnich <rminnich@gmail.com> wrote:

> since .. came up, nobody mentioned it yet,
> https://plan9.io/sys/doc/lexnames.pdf
>
> but the subject line is "moving directories" and, as pointed out, on
> old school file systems with hard links, it's not a move, just an ln,
> more or less. And in the easy case, it's easy; and in the not so easy
> case, it can be impossible ...
>
> On newer systems, which don't have such notions as hard links, it's a
> different game, and on many, it may not be possible; dircp to the
> rescue, with all the pain that implies (doesn't go well when you are
> in CA and the server is in NJ, trust me :-)
>
> But this brings up a question I forgot: what was the last Unix version
> that let users make arbitrary links, such that the file system was no
> longer a DAG? I recall in v6 days hearing that earlier Unix allowed
> this, and that cleanup (via icheck and friends) got to be near
> impossible.
>
> On Wed, Dec 29, 2021 at 9:50 AM Brantley Coile <brantley@coraid.com>
> wrote:
> >
> > Plan 9 can't move directories with mv. I will only change the name of
> them.
> > (If this is the question. I was only half paying attention to the
> thread. Sorry)
> >
> >         --bwc
> >
> > cessna% mkdir dira
> > cessna% mkdir dirb
> > cessna% touch dira/a
> > cessna% touch dirb/b
> > cessna% mv dira dirb
> > mv: can't remove ./dirb: remove -- directory not empty
> >
> > To move contents of directories we use dircp.
> >
> > cessna% man dircp
> >
> >      TAR(1)                                                     TAR(1)
> >
> >      NAME
> >           tar, dircp - archiver
> >
> >      SYNOPSIS
> >           tar key [ file ... ]
> >
> >           dircp fromdir todir
> >
> >      DESCRIPTION
> >           Tar saves and restores file trees.  It is most often used to
> >           transport a tree of files from one system to another.  The
> >
> >         ...
> >                and .tz.  If no extension matches, gzip is used.  The z
> >                flag is unnecessary (but allowed) when using the t and
> >                x verbs on archives with recognized extensions.
> >
> >      EXAMPLES
> >           Tar can be used to copy hierarchies thus:
> >
> >                @{cd fromdir && tar c .} | @{cd todir && tar xT}
> >
> >           Dircp does this.
> >
> >      SOURCE
> >           /sys/src/cmd/tar.c
> >           /rc/bin/dircp
> >
> >      SEE ALSO
> >           ar(1), bundle(1), tapefs(4), mkfs(8)
> >
> >
> >
> > cessna%
> > > On Dec 29, 2021, at 12:38 PM, Clem Cole <clemc@ccc.com> wrote:
> > >
> > > At the risk of kicking a dead horse too much ...
> > >
> > > On Wed, Dec 29, 2021 at 12:14 PM <arnold@skeeve.com> wrote:
> > > Plan 9 eventually did something like this. I don't remember the
> details.
> > > Arnold - point taken but ... unfortunately, I think that the
> comparison is a little difficult because Plan9's concepts of namespaces is
> a tad different than UNIX's.  But I'll let Rob or Ken comment as they lived
> and developed both systems.
> > >
> > > FWIW: An object store is something that retains information after the
> processes that operates on it complete - i.e. its a static entity.  Links
> were (are) also a static concept.   Late binding to names (like symlinks)
> are a dynamic (runtime idea).  Bakul points out that by using the per
> process u area, the dynamic context can be retained.  The observation is
> that .. (like symlinks) tend to be a runtime (dynamic) notion, although I'm
> not sure how you keep consistency in the static FS if you don't store the
> link in the inode.  As someone that did, I suggest - try writing fsck if
> you are using dynamic content.   How do you know?  I'd still claim it is
> the same issue.
> > >
> > > Anyway, I suppose that like context sensitive symlinks (which I sorely
> miss), you could do this and keep a list of the N inodes for the N parents
> and then like CDSL's keep the one you used to get there in the u area so
> that .. picks the proper one on the way out and you can still have the
> static notion which something like fsck can check off line.
> >
>

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