36 Understanding the IBM TotalStorage Open Software Family
In this book, we use the following terms:
Host Any server that is using storage and runs applications for
Agent A hardware or software component installed on a host to
virtualize its view of storage or to execute a specific
storage management function for that particular host.
Server A component of the storage network that provides a
service to its clients. This can be a host server or a
Client Something requesting a service from a server. This can
be a client to an application on a host or a host when
requesting service from a virtualization server.
Controller A hardware or software component that controls access
and usage of the storage resources it manages.
Now to save you from total confusion, let us agree that client and server are
interchangeable terms and that the relationship is defined by the interaction at a
particular time. We will use illustrations to clarify these interactions and qualify
the terms, where appropriate.
1.6.3 Levels of virtualization
We will define the different levels at which virtualization can be achieved in a
storage network, as illustrated in Figure 1-11 on page 37.
Chapter 1. Understanding storage concepts 37
Figure 1-11 Virtualization levels
Abstraction at the server level is accomplished by the volume management of
the operating systems on the servers. Some operating systems, like OS/390®
with DFSMS, have a high level of abstraction, whereas others still present, to the
user, volumes that are directly related to physical disks. At first sight, increasing
the level of abstraction on the server seems well suited for environments without
storage networks, but this can be vitally important in storage networks too, if we
look at the security and sharing issues.
On the SAN Fabric level, virtualization can enable the independence of storage
pools from heterogeneous servers. A single management interface can be
implemented to manage disparate storage systems without impacting the
servers. Several approaches are possible, and we will go into more detail on this.
At the storage device level, several virtualization techniques have already been
tried, ranging from striping and redundant array of independent disks (RAID) to
sophisticated volume management in the Enterprise Storage Server (ESS) or
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