Datasheet

Part I ✦ Laying the Foundation

Availability

The availability of information refers to the information’s accessibility when required regard-

ing uptime time, locations, and the availability of the data for future analysis. Recovery,

redundancy, archiving, and network delivery all affect availability.

The system requirements drives availability, which is often described in terms of 9s. Five 9s

means the system is available 99.999 percent of the required time, as shown in Table 1-3.

Table 1-3: The Chart of Nines

Percent Uptime Downtime Description

99.99999 364d 23h 59m 56s 000d 00h 00m 03s “7 nines”

99.9999 364d 23h 59m 29s 000d 00h 00m 31s “6 nines”

99.999 364d 23h 54m 45s 000d 00h 05m 15s “5 nines”

99.99 364d 23h 07m 27s 000d 00h 52m 33s “4 nines”

99.95 364d 19h 37m 12s 000d 04h 22m 48s -

99.9 364d 15h 14m 24s 000d 08h 45m 36s “3 nines”

99.8 364d 15h 14m 24s 000d 17h 31m 12s -

99.72603 364d 00h 00m 00s 001d 00h 00m 00s exactly 1 day

99.5 363d 04h 12m 00s 001d 19h 48m 00s -

99 361d 08h 24m 00s 003d 15h 36m 00s “2 nines”

98 357d 16h 48m 00s 007d 07h 12m 00s -

97 354d 01h 12m 00s 010d 22h 48m 00s -

Chapter 36, “Recovery Planning,” and Chapter 52, “Providing High Availability,” both provide

details on SQL Server 2005’s availability features.

Redundancy

Redundancy is the identification of possible points of failure and avoiding or reducing the

effects of the failure by providing a secondary solution. For some disciplines, redundancy

suggests waste, but for data stores, redundancy is a good thing.

A warm standby server is a copy of the database on another server ready to go live at a

moment’s notice. Typically, this method uses log shipping to move the latest transaction to

the warm standby server. A clustered server is an expensive hardware link between two

servers such that when the primary server goes down, the backup server is immediately

online.

At some point, the best hardware will fail. If the information is extremely critical to the organi-

zation, management may decide that it’s too expensive (in lost time to the organization) to

Cross-

Reference

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