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Re: Hurd and Unix/Linux and Plan9 features

From: olafBuddenhagen
Subject: Re: Hurd and Unix/Linux and Plan9 features
Date: Sat, 3 Feb 2007 17:11:00 +0100
User-agent: Mutt/1.5.13 (2006-08-11)


On Fri, Feb 02, 2007 at 12:34:16PM +1300, Shams wrote:

> 1. Will hurd still retain the symbolic link and hard link concept of
> Unix/Linux?

Yes, the Hurd supports symlinks and hardlinks in the traditional way, as
well as an additional link type: Firmlinks.

> * Also will it support hard links for directories?

The current implementation doesn't support them, but this might change.

> * Can tranlators be symbolic linked or hard linked?

Sure. A translator sits on some inode, and you can always link this
inode like any other file.

> * Currently in *Nix one has to use the find command to find a list of
> all symbolic links and hard links. I am wondering if this will be made
> much more easier in hurd?

There are no plans for anything like that.

> 2. Will it support the Linux LVM concept?

The Hurd allows for complex stores, which -- amongs ome other
possibilities -- can contain a concatenation of several individual
stores. This way you can use several partitions for one filesystem. I'm
not sure how Linux LVM works exactly, but I think the concept is

There is presently no complatibility with Linux LVM, but I guess it
shouldn't be too hard to implement, if someone is interested in that.

> 3. Also what features does Hurd borrow or enhance from the Plan 9 OS.
> I mean for example Plan 9 abstracts everything as files. Will/does
> Hurd support such concept (its sees everthing as files) or is this
> just achievabal by existing or writing custom translators?

I don't know whether it is actually borrowed (the Hurd is nearly as old
as Plan9), but the translator concept is actually quite similar to Plan9
FS servers -- both alow normal user programs to export filesystems. The
technical implementation is pretty different though, so there are a
number differences in what you can do.

The Plan9 FS servers are specifically tailored for building a system
with FS interfaces for everything. Hurd translators are not specifically
designed for that, so they might be a little less convenient for this
particular use case. (Haven't investigated the detals.) They are more
generic/powerful, on the other hand.

Not everyone likes the idea of doing everthing with standard FS
interfaces. The translator concept allows implementing any kind of
interface, not only normal filesystems. This way it is possible to have
services that are accessible through the filesystem tree, but implement
custom semantics. (Either completely new semantics, or extensions to the
standard filesystem semantics.) There are a few things in the Hurd that
make use of this possibility.


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