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Chapter 2 HPSS Planning
92 September 2002 HPSS Installation Guide
Release 4.5, Revision 2
to determine HPSS hardware requirements and determine how to configure this hardware to
provide the desired HPSS system. The process of organizing the available hardware into a desired
configuration results in the creation of a number of HPSS metadata objects. The primary objects
created are classes of service, storage hierarchies, and storage classes.
A Storage Class is used by HPSS to define the basic characteristics of storage media. These
characteristics include the media type (the make and model), the media block size (the length of
each basic block of data on the media), the transfer rate, and the size of media volumes. These are
the physical characteristics of the media. Individual media volumes described in a Storage Class
are called Physical Volumes (PVs) in HPSS.
Storage Classes also define the way in which Physical Volumes are grouped to form Virtual
Volumes (VVs). Each VV contains one or more PVs. The VV characteristics described by a Storage
Class include the VV Block Size and VV Stripe Width.
A number of additional parameters are defined in Storage Classes. These include migration and
purge policies, minimum and maximum storage segment sizes, and warning thresholds.
An HPSS storage hierarchy consists of multiple levels of storage with each level represented by a
different Storage Class. Files are moved up and down the storage hierarchy via stage and migrate
operations, respectively, based upon storage policy, usage patterns, storage availability, and user
requests. If data is duplicated for a file at multiple levels in the hierarchy, the more recent data is at
the higher level (lowest level number) in the hierarchy. Each hierarchy level is associated with a
single storage class.
Class of Service (COS) is an abstraction of storage system characteristics that allows HPSS users to
select a particular type of service based on performance, space, and functionality requirements.
Each COS describes a desired service in terms of such characteristics as minimum and maximum
file size, transfer rate, access frequency, latency, and valid read or write operations. A file resides in
a particular COS and the COS is selected when the file is created. Underlying a COS is a storage
hierarchy that describes how data for files in that class are to be stored in the HPSS system. A COS
can be associated with a fileset such that all files created in the fileset will use the same COS.
A file family is an attribute of an HPSS file that is used to group a set of files on a common set of
tape virtual volumes. HPSS supports grouping of files only on tape volumes. In addition, families
can only be specified by associating a family with a fileset, and creating the file in the fileset. When
a file is migrated from disk to tape, it is migrated to a tape virtual volume assigned to the family
associated with the file. If no family is associated with the file, the file is migrated to the next
available tape not associated with a family (actually to a tape associated with family zero). If the file
is associated with a family and no tape VV is available for writing in the family, a blank tape is
reassigned from family zero to the file’s family. The family affiliation is preserved when tapes are
repacked.
The relationship between storage class, storage hierarchy, and COS is shown in Figure 2-2.