[fhs-discuss] static sharable files

Steve Langasek vorlon at debian.org
Tue May 17 01:43:01 PDT 2011


On Mon, May 09, 2011 at 09:33:48AM +0200, Tollef Fog Heen wrote:
> I think
> http://www.freedesktop.org/wiki/Software/systemd/separate-usr-is-broken
> is relevant in this context.

Frankly, this says more about the willingness of certain implementors to
conform to the FHS than it does about whether the FHS's rules for /usr vs. /
are broken.

I find the idea that pulseaudio can't function correctly without /usr on the
root partition a particularly mirthful assertion.

> Separate /usr made sense back when drives were small and disk space was
> expensive, but in the vast majority of cases today, having /usr on the
> root file system is no real burden.  Not having it on the root file
> system means more brittle setups and trying to share /usr between
> installations can easily lead to maintenance headaches.

It may be that one of the original justifications for separate /usr, NFS
sharing, no longer makes sense, but it's too late to put humpty dumpty back
together.  There are too many *perfectly functional* systems that rely on
/usr as a separate partition, and we should not be forcing their admins to
rework their disk layouts just because some implementors ignore the FHS.

On Mon, May 09, 2011 at 10:38:42AM +0100, Roger Leigh wrote:

> http://lists.debian.org/debian-devel/2011/01/msg00152.html

> In this message, I've detailed various historical use cases for a
> separate /usr, together with brief pros/cons/rationale.  On a modern
> Linux system, most of these make zero sense, and it is preferable to
> have them on the same filesystem.

The only one from your list that "makes zero sense" is sharing /usr over
NFS.  (Actually, 6, 7, 8, 9 don't make sense either... but they didn't make
sense *historically* and I've never seen those used as a justification for
/usr.)

It's fine that you prefer to put /usr on the same filesystem.  Nevertheless
there are reasons why, in some circumstances, users will choose not to do
this.  The FHS has always allowed for this, and there is insufficent
rationale to break support for this now.

To be clear, I'm *also* in favor of ensuring that /usr as a symlink to / is
well-supported.  But I'm a user with encrypted / and unencrypted /usr, and
it matters to me that there is no risk of filesystem corruption causing
shards of LibreOffice to be shotgunned through my critical system libraries. 
Oh, and pulseaudio, dbus, plymouth, cups, and NetworkManager all work
perfectly well for me.

> From the FHS POV, I would like to suggest these changes:

> ? "/usr is shareable, read-only data"
>   - Remove the "shareable" qualifier.  With a package manager such as
>     dpkg or rpm etc., it does not make any sense to share /usr since
>     the content is managed as a whole with the other contents of /,
>     including conffiles.

Not everyone uses dpkg or rpm.  I think the "shareable" should be retained.

>   - Even when you don't have a package manager, you still have the issue
>     with the configuration files not being shared (as pointed out
>     elsewhere in this thread).

But if they're configuration files, there's no reason to think they *should*
be shared, either...?

On Mon, May 09, 2011 at 01:36:49PM +0200, Tollef Fog Heen wrote:

> The alternative is to move a significant chunk of what's in /usr to /
> for services to work correctly.  How big this chunk is, I don't know.

On an Ubuntu desktop system, excluding /lib/modules and /lib/firmware (which
can vary widely in size), my /lib+/bin+/sbin is 53MB, and that includes
about 7MB of libraries for ABIs not actually needed for booting.  This
includes all the libraries needed to bring up a network connection via
NetworkManager, as well as the rendering libraries to provide graphical boot
with plymouth.

Compare that with /usr, which sits at 6.1GB on my system.

This does actually miss a few pieces that rightfully belong on /; in
particular, the various components needed to run kerberized NFS are located
under /usr (libs and daemons), so you can't put /usr on kerberized NFS
(shared or otherwise).  Fixing this has been a low priority because, as
Roger points out, there isn't actually a lot of interest in NFS-mounted
/usr.  But it could be done, at a cost of surely no more than another 100MB.

Cheers,
-- 
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
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