Some companies are willing to schedule regular maintenance windows
for major systems and networking work in return for better availability during
normal operations. Depending on the size of the site, this could be one
evening and night per month or perhaps from Friday evening to Monday
morning once a quarter. These maintenance windows are necessarily very
intense, so consider the capacity and well-being of the system administration
staff, as well as the impact on the company, when deciding to schedule them.
SAs often like to have a maintenance window during which they can take
down any and all systems and stop all services because it reduces complexity
and makes testing easier. It’s difficult to change the tires while the car is
driving down the highway. For example, in cutting email services over to
a new system, you need to transfer existing mailboxes, as well as switch
the incoming mail feed to the new system. Trying to transfer the existing
mailboxes while new email arrives and yet ensure consistency is a very tricky
problem. However, if you can bring email services down while you do the
transfer, it becomes a lot easier. In addition, it is a lot easier to check that the
system is working correctly before you turn the mail feed and the read access
on again than it is to deal with having dropped or bounced mail if something
didn’t work quite right with the live cutover