Multiarch in Oneiric for developers

[Steve Langasek]

Hi David,

On Fri, Aug 26, 2011 at 12:00:38AM +0200, David Henningsson wrote:
> On 2011-08-16 20:46, Steve Langasek wrote:
> >As part of the work to eliminate ia32-libs in favor of multiarch library
> >installation
> >(https://blueprints.launchpad.net/ubuntu/+spec/foundations-o-multiarch-next-steps),
> >i386 sources have been enabled by default on amd64 systems in oneiric.  For
> >new installs, this is done by default in alpha-3 and above; and as of today,
> >update-manager also ensures that i386 multiarch support is enabled by
> >default on upgrade from natty.

> I'm sure this is all great and something many people have been
> waited eagerly for, but for my own part I must say I have missed the
> newbie introduction wiki/blog, or "Multiarch for dummies". [1]

Yes, the information so far has not been targeted at end users because there
hasn't been anything for users to directly *use* until now.  I'm intending
to put something in the beta release notes about this.

I also gave a talk at DebConf about multiarch (slides[1], video[2]), which
briefly covers the "why bother?" aspect among other things; and there's
<http://wiki.debian.org/Multiarch/TheCaseForMultiarch> which tries to lay
out the problem space multiarch is designed to address.

As to your specific questions, answers are inline below:

> 1) If I'm an Ubuntu end user who likes to watch youtube videos, what
> does multiarch mean to me and what advantages will I notice?

The end user is not likely to directly notice any advantages with respect to
youtube videos - or at least, not ones that are obviously attributable to
multiarch.  What multiarch does is let us make the ia32-libs compat package
obsolete, rendering an entire class of bugs irrelevant: because the 32-bit
libraries are now delivered to the amd64 user the same way as they are to
the i386 user, we will no longer have bugs (including security bugs)
remaining unfixed in an unwieldly all-or-nothing compatibility package, nor
will we have bugs *caused* by stale copies of libraries in ia32-libs.  (Ask
any of the X guys how much fun it is to try tracking down the out-of-date
copy of mesa in ia32-libs that mysteriously causes amd64 desktops to hang.)

Beyond the removal of ia32-libs, which is still in progress, multiarch will
also allow us to make many *more* 32-bit libraries available on the 64-bit
desktop, instead of a hand-picked few libraries used by certain common
applications.  As a result, 32-bit software will in the long run be more
usable on 64-bit systems, because we have a solution that scales (instead of
ia32-libs which doesn't); and because of this improved compatibility, more
users will be able to take fuller advantage of their 64-bit systems by
running the amd64 architecture without fear that their 32-bit programs won't
work.

And the libraries that are converted for multiarch will work better than
they did in ia32-libs.  Longstanding (recurring) bugs related to search
paths for gtk themes and other plugins are resolved by switching to
multiarch.

> 2) As I understand Multiarch is a way to run i386 binaries on an
> amd64 system. [2] What are the typical i386 binaries you want to run
> on amd64?

The typical ones for which we have multiarch library support in oneiric are
skype and flash.  There are three other 32bit-only closed-source packages in
the Canonical partner archive that are not yet installable via multiarch:
acroread, adobeair, and adobereader-deu.  Wine, Google Earth, and ICA Client
are other common 32-bit-only packages.  There's plenty of other
closed-source software for Linux that's also affected.

> 3) If I'm a packager of a random library, does multiarch affect my
> packaging? Is there anything I should be aware of or should it "just
> work"? (What if I use debhelper 7, or cdbs, etc?)

http://wiki.debian.org/Multiarch/Implementation

Hope that helps,
-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org

[1] http://people.debian.org/~vorlon/multiarch-six-years-on.pdf
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