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Chapter 1 HPSS Basics
HPSS Installation Guide September 2002 23
Release 4.5, Revision 2
• Virtual Volumes. A virtual volume is used by the Storage Server to provide a logical
abstraction or mapping of physical volumes. A virtual volume may include one or more
physical volumes. Striping of storage media is accomplished by the Storage Servers by
collecting more than one physical volume into a single virtual volume. A virtual volume is
primarily used inside of HPSS, thus hidden from the user, but its existence benefits the user
by making the user’s data independent of device characteristics. Virtual volumes are
organized as strings of bytes up to 2
64-1
bytes in length that can be addressed by an offset
into the virtual volume.
• Storage Segments. A storage segment is an abstract storage object which is mapped onto
a virtual volume. Each storage segment is associated with a storage class (defined below)
and has a certain measure of location transparency. The Bitfile Server (discussed in Section
1.3.2) uses both disk and tape storage segments as its primary method of obtaining and
accessing HPSS storage resources. Mappings of storage segments onto virtual volumes are
maintained by the HPSS Storage Servers (Section 1.3.2).
• Storage Maps. A storage map is a data structure used by Storage Servers to manage the
allocation of storage space on virtual volumes.
• Storage Classes. A storage class defines a set of characteristics and usage parameters to
be associated with a particular grouping of HPSSvirtual volumes. Each virtual volume and
its associated physical volumes belong to a single storage class in HPSS. Storage classes in
turn are grouped to form storage hierarchies (see below). An HPSS storage class is used to
logically group storage media to provide storage for HPSS files with specific intended
usage, similar size and usage characteristics.
• Storage Hierarchies. An HPSS storage hierarchy defines the storage classes on which
files in that hierarchy are to be stored. A hierarchy consists of multiple levels of storage,
with each level representing a different storage class. Files are moved up and down the
hierarchy via migrate and stage operations based on usage patterns, storage availability,
and site policies. For example, a storage hierarchy might consist of a fast disk, followed by
a fast data transfer and medium storage capacity robot tape system, which in turn is
followed by a large data storage capacity but relatively slow data transfer tape robot
system. Files are placed on a particular level in the hierarchy depending upon the
migration levels that are associated with each level in the hierarchy. Multiple copies are
controlled by this mechanism. Also data can be placed at higher levels in the hierarchy by
staging operations. The staging and migrating of data is shown in Figure 1-1.
• Class of Service (COS). Each bitfile has an attribute called Class Of Service. The COS
defines a set of parameters associated with operational and performance characteristics of
a bitfile. The COS results in the bitfile being stored in a storage hierarchy suitable for its
anticipated and actual size and usage characteristics. Figure 1-2 shows the relationship
between COS, storage hierarchies, and storage classes.