Hub/Switch Installation Guide

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
collectingmorethan onephysicalvolume intoa singlevirtualvolume. Avirtual volume is
primarilyusedinsideofHPSS,thushidden fromtheuser,butitsexistencebenefitstheuser
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. Astoragesegment isan abstractstorage objectwhichis mappedonto
a virtual volume. Each storage segment is associated with a storage class (defined below)
and has a certainmeasureof locationtransparency. TheBitfile Server(discussed inSection
1.3.2) uses both disk and tape storage segments as its primary method of obtaining and
accessingHPSSstorage resources.Mappingsof storagesegmentsontovirtual 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
beassociatedwithaparticulargroupingofHPSSvirtualvolumes.Eachvirtualvolumeand
its associatedphysical volumes belongto a single storage classin HPSS. Storageclasses in
turn are groupedto form storagehierarchies (seebelow). An HPSSstorage class is usedto
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 hierarchymightconsist of a fastdisk, followedby
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 thehierarchy 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 aset of parametersassociated with operationaland performance characteristicsof
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.