== Some _yum_ tricks with _distro-sync_ and _--releasever_

Suppose, not entirely hypothetically, that you have just upgraded your
Fedora 13 workstation to Fedora 14 (with the latest updates) only to
find out that your window manager now causes the X server to crash.
Seeing as this renders the system unable to start your environment, you
would like to find a solution by trying older versions of the X server.

As it happens, _yum_ has added some interesting options since I
last read through its manual page; in fact, I found out about
_--releasever_ and _distro-sync_ from the [[Yum upgrade FAQ
http://fedoraproject.org/wiki/YumUpgradeFaq]].  Today I exploited them
to downgrade packages the easy way, or at least what I thought of as the
easy way.

(For details on both of these options, see the yum manpage. If your
yum manpage doesn't cover them, you can't use them anyways.)

First off, I tried downgrading the X server to the original release
version instead of the updated version:

> _yum --disable-repo updates distro-sync xorg-x11-server-Xorg_

With the updates repository disabled, _distro-sync_ will only see the
base release version and will (if necessary) revert the package back to
that version. I believe but haven't tested that '_yum downgrade_' will
do the same thing but in a simpler way.

(These two approaches wouldn't be quite the same if Fedora updates kept
around more than one version of packages.)

When that didn't work, I decided that I wanted to revert all the way
back to the latest Fedora 13 version of the X server (which I knew
worked, because I had been running it):

> _yum --releasever=13 distro-sync xorg-x11-server-Xorg_

I was actually kind of surprised that this worked, but it did.  Normally
one uses _--releasever_ to go forward to the next distribution, but it
works backwards too, and with the distribution set back to Fedora 13
_distro-sync_ will thus downgrade the X server to the latest Fedora
13 version. I would definitely recommend running '_package-cleanup
--problems_' after doing this stunt.

(As it happened I needed to also revert a bunch of other X server
related packages in order to get the Fedora 13 X server to not lock
up. The result runs my window manager, though, and so I can now actually
start my environment on Fedora 14 and be productive.)

As a side note, one reason that I latched on to _distro-sync_ for doing
this is that it was easy to reason about how it would behave.  Given
a specific repository configuration it was easy to see what the end
result of _distro-sync_ be; the only challenge was arranging the right
repository configuration. (Of course this was made easier because what I
wanted to do could be expressed this way; something like 'revert to the
previous update version' would have been completely impossible.)