[CentOS] updating without rebooting

Fri Mar 25 22:53:31 UTC 2011
Devin Reade <gdr at gno.org>

Slightly OT, but I have more than one pacemaker/corosync cluster where one
of the main reasons to use a cluster (in addition to the availability
aspect) is to be able to perform running updates without affecting the
user base.  As in:

  1. All services running on node A.
  2. Update node B.
  3. Cut services over to node B.
  4. Test services.
  5. Update node A.
  6. Cut services back to node A.
  7. Test services.

In these particular cases, it was deemed acceptable that the time to
cut a service over from one node to another did not require a special
maintenance window (including the time to revert back to the non-updated
node should there be a problem with the update).  Thus, we're able
to apply updates at our leisure and not end up with a long time between
update and reboot.

It sure makes keeping a production system current a whole lot easier.

(Depending on the service, cutover times range from about 5-60 seconds
per service).