Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!motcsd!xdos!doug
From: doug@xdos.UUCP (Doug Merritt)
Newsgroups: comp.sys.amiga.tech
Subject: Re: yet another 1.4 request
Summary: Playing devil's advocate: "pro's of hard links"
Message-ID: <398@xdos.UUCP>
Date: 30 Jun 89 16:30:36 GMT
References: <8906300342.AA12274@postgres.Berkeley.EDU>
Reply-To: doug@xdos.UUCP (Doug Merritt)
Organization: Hunter Systems, Mountain View CA (Silicon Valley)
Lines: 90

In article <8906300342.AA12274@postgres.Berkeley.EDU> dillon@POSTGRES.BERKELEY.EDU (Matt Dillon) writes:
>
>	Soft links are better than hard links period.
>
>	* Removing a softlink removes the softlink, NOT the file it is pointing
>	  to

And yet there are times when you want to set things up so that removing
the softlink *does* remove the file itself, if the file (inode) reference
count goes to zero. This can't happen with soft links, where only one
of the file references is the real file.

>	* You can create soft links to non existant (or not yet existant) paths

And yet there are times when you *want* a complaint if the file doesn't
exist yet, or to be able to preserve a file simply by linking to it.
With hard links, you can link to someone else's file, and they can delete
it when they're done with it without worry, because you can do the same
independently. With soft links you'd have to take the extra steps of
copying the file somewhere safe, replacing the original sole definition
of the file with a soft link, and then eventually coordinating with 
everyone to see when and whether the file body should be deleted. Or in
practice forgetting about it, everybody deletes their soft links, and
the file needlessly takes up disk space.

Although this can come up with a single person & multiple projects, clearly
it is more likely to in a multiuser environment. But since we can safely
assume that Amiga's will never be used in such environments, like LANs,
we can ignore this, eh? :-)

>	* You can have soft links across filesystems

On Unix, using hard links to files on the same file system
gives a performance advantage over soft links, since opening the
hardlink immediately gives the inode reference, whereas soft links
require a second path search to find the real inode. So far it seems
to me that the same could be true on AmigaDos.

>	* You can have soft links to devices

Bogus, on both AmigaDOS and on Unix you can hard link to devices. On
Unix you just do it; "ln /dev/tty ~doug/my_tty" works. On AmigaDOS
you can create a second device list entry, or do an assign. And if
AmigaDOS had a consistent name space, where device names *really* lived
inside a file system, then you could just plain link them, too.

>	* It doesn't require any hacking on low level FS structures

Implementation detail.

>	Hard links are dangerous because:
>
>	* They are not easily supportable under AmigaDOS due to the filename
>	  being in the file header (inode) instead of the directory entry
>	  (what directory entry!).

Implementation problems are a secondary issue after consideration of
desirability.

>	* backup programs would have to deal with circularities and, even when
>	  found, since there is no distinction between a master or slave link
>	  the user can get very very confused.  At least with soft links you
>	  know where you stand.

The circularites don't come about on Unix because you are prevented
from hard linking to directories. Multiple backups are prevented on
AmigaDOS by the archived protection bit. I grant you the confusion, except
that even soft links can lead to a great deal of confusion, as we find
out daily on Sun 3.5. (And on Sun 386i 4.0, they lead to nothing *but*
confusion due to brain damaged system setup. But that's Sun's problem.)

>	* Hardlinks look like normal files or directories... all the more
>	  confusing.

Harlinks *are* normal files (but again, not directories). The only
point of confusion with hardlinks is that you may not know where
other copies of a file are, but "find . -inum ..." can fix that in
the rare cases where it matters. In any event, it's not clear that
a small amount of confusion means that they should be tossed altogether.
Especially considering that soft links really are *more* confusing. I
hate to do "cd directory; cd .." and discover that I haven't returned
to where I started. This never happens with hard links.
	Doug

P.S. I'm ignoring the limited exceptions to the rule that you can't
hardlink to Unix directories because those exceptions don't seem
important in this context.
-- 
Doug Merritt		{pyramid,apple}!xdos!doug
Member, Crusaders for a Better Tomorrow		Professional Wildeyed Visionary