Datasheet

2
|
CHAPTER 1 What Kind of Protection do You need?
happening, if possible. These two alternatives, highly available disk or nightly tape, provided
two extremes where your data loss was measured at either zero or in numbers of days.
The concept of data availability was a misnomer. Your data either was available from disk or
would hopefully be available if the restore completed, resulting in more a measure of restore
reliability than an assurance of productive uptime. That being said, let’s explore the two sides of
today’s alternatives: data availability and data protection.
Overview of Availability Mechanisms
Making something more highly available than whatever uptime is achievable by a standalone
server with a default configuration sounds simple — and in some ways it is. It is certainly easier to
engage resiliency mechanisms within and for server applications today than it was in the good ol’
days. But we need to again ask the question “What are you solving for?” in terms of availability. If
you are trying to make something more available, you must have a clear view of what might break
so that something would be unavailable — and then mitigate against that kind of failure.
In application servers, there are several layers to the server — and any one of them can break
(Figure 1.1).
Figure 1.1 isn’t a perfect picture of what can break within a server. It does not include the infra-
structure — such as the network switches and routers between the server and the users’ worksta-
tions. It doesn’t include the users themselves. Both of these warrant large sections or books in their
own right. In many IT organizations, there are server people, networking people, and desktop peo-
ple. This book is for server people, so we will focus on the servers in the scenario and assume that
our infrastructure is working and that our clients are well connected, patched, and knowledgeable,
and are running applications compatible with our server.
For either data protection or data availability, we need to look at how it breaks — and then
protect against it. Going from top to bottom:
If the logical data breaks, it is no longer meaningful. This could be due to something as
•u
dire as a virus infection or an errant application writing zeros instead of ones. It could also
be as innocent as the clicking of Save instead of Save As and overwriting your good data
with an earlier draft. This is the domain of backup and restore — and I will cover that in
the “Overview of Protection Mechanisms” section later in this chapter. So, for now, we’ll
take it off the list.
In the software layers, if the application fails, then everything stops. The server has good
•u
data, but it isn’t being served up to the users. Chapters 5 through 9 will look at a range of
technologies that offer built-in availability. Similarly, if the application is running on an
Figure 1.1
Layers of a server
Logical Data
Application Software
Operating System
File System
Server Hardware
Storage Hardware
572146c01.indd 2 6/23/10 5:42:19 PM