Capacity Planning - The Practice of Cloud System Administration - page 370

capacity acrossmanyfailuredomainsreducestheadditional capacity requiredforhandling
the resiliency requirements, which is the most cost-effective way to provide this extra ca-
pacity. For example, if a service runs in one datacenter, a second datacenter is required to
provide the additional capacity, about 50 percent. If a service runs in nine datacenters, a
tenth is required to provide the additional capacity; this configuration requires only 10 per-
cent additional capacity.
As discussed in Section 6.6.5 , the gold standard is to provide enough capacity for two
datacenterstobedownatthesametime.Thispermitsonetobedownforplannedmainten-
ance while the organization remains prepared for another datacenter going down unexpec-
tedly. Appendix B discusses the history of such resilient architectures.
18.1.6 Timetable
Most companies plan their budgets annually, with expenditures split into quarters. Based
on your expected normal growth and planned growth bursts, you can map out when you
need the resources to be available. Working backward from that date, you need to figure
out how long it takes from “go” until the resources are available.
How long does it take for purchase orders to be approved and sent to the vendor? How
long does it take from receipt of a purchase order until the vendor has delivered the goods?
How long does it take from delivery until the resources are available? Are there specific
tests that need to be performed before the equipment can be installed? Are there specific
changewindowsthatyouneedtoaimfortoturnontheextracapacity? Oncetheadditional
capacity is turned on, how long does it take to reconfigure the services to make use of it?
Using this information, you can provide an expenditures timetable.
Physical services generally have a longer lead time than virtual services. Part of the
popularity of IaaS and PaaS offerings such as Amazon's EC2 and Elastic Storage are that
newly requested resources have virtually instant delivery time.
Itisalwayscost-effectivetoreduceresourcedeliverytimebecauseitmeanswearepay-
ing for less excess capacity to cover resource delivery time. This is a place where automa-
tion that prepares newly acquired resources for use has immediate value.