Subject: path component substitutions (was diskless booting)
To: Stefan Grefen <>
From: Ty Sarna <>
List: tech-kern
Date: 03/15/1994 04:27:33
Stefan Grefen wrote:
> Please not again.  I admit this stuff is useful, but I hate to think
> about the consequences for let's say a backup. 

Huh? There are no consequenses for a backups. I'm not sure you have an
accurate understanding of how the @substitiion stuff works.

> How is xy.@sys handled? As a symlink it wouldn't break anything, but as

xy.@sys where? You mean a directory entry called "xy.@sys"? A symlink
pointing to xy.@sys? or accessing xy.@sys and getting xy.arch_os123?

> a normal entry in the filesystem you would backup one of these twice, or as
> a link (but that would introduce links to directories and I'm sure these
> will break at least find (it would traverse it twice) ...).

You don't have to back up twice. If your /usr/local looks like:

bin -> @sys/bin

if you do back that up, there would be no detectable difference between
doing that on a system with @subs and doing it on one without, unless
you told the backup to follow symlinks.

> A more generic solution would be to implemented something like SUN's TFS.
> NETBSD's lofs could be good starting point for that.

That's no more generic. Either way, you're required to add something to
the system that currently doesn't exist, right? :-)

Besides, the @substitutions are in wide use in AFS and in OSF's DFS, and
work very very well in practice.  They've also been implemented and used
on a much wider range of systems than TFS has, since TFS is a
Sun-specific thing.

> PS. I volunteer for the TFS code as soon as my logical volume manager is 
> finished. (that should be in the near future (2-4 weeks) as I get a decent 
> notebook one of the next days)

Great! TFS is useful for many things, I just don't think this is one of
them (particularly the original case that restarted this discussion --
TFS wouldn't work there).

I administered a small chunk of AFS space in a multiplatform environment
for a while, and absolutely fell in love with @substitutions.  I've
never found anything else remotely as clean and easy.  It requires no
modifications to packages to install them (unlike some schemes such as
depot), and doesn't require users to have special privledges to edit
fstab or automounter configurations or whatever.  This means users can
easily set up their own private multi-arch trees, say in ~/bin for their
own private programs.  In fact there is no setup on configuration
neccesarry at all...  "just do it", and it works the way you want.  In
fact, for all of AFS's fancy features, the simple idea of @substitutions
is, IMHO, one of AFS/DSF's best features. 

At any rate, I didn't mean to stir up so much discussion... I still want
to hear discussion on the diskless boot architecture. :-)

Ty Sarna                 "As you know, Joel, children have always looked        up to cowboys as role models. And vice versa."