Chapter 2 HPSS Planning92 September 2002 HPSS Installation GuideRelease 4.5, Revision 2to determine HPSS hardware requirements and determine how to configure this hardware toprovide the desired HPSS system. The process of organizing the available hardware into a desiredconfiguration results in the creation of a number of HPSS metadata objects. The primary objectscreated are classes of service, storage hierarchies, and storage classes.A Storage Class is used by HPSS to define the basic characteristics of storage media. Thesecharacteristics include the media type (the make and model), the media block size (the length ofeach basic block of data on the media), the transfer rate, and the size of media volumes. These arethe physical characteristics of the media. Individual media volumes described in a Storage Classare called Physical Volumes (PVs) in HPSS.Storage Classes also define the way in which Physical Volumes are grouped to form VirtualVolumes (VVs). Each VV contains one or more PVs. The VV characteristics described by a StorageClass include the VV Block Size and VV Stripe Width.A number of additional parameters are defined in Storage Classes. These include migration andpurge 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 adifferent Storage Class. Files are moved up and down the storage hierarchy via stage and migrateoperations, respectively, based upon storage policy, usage patterns, storage availability, and userrequests. If data is duplicated for a file at multiple levels in the hierarchy, the more recent data is atthe higher level (lowest level number) in the hierarchy. Each hierarchy level is associated with asingle storage class.Class of Service (COS) is an abstraction of storage system characteristics that allows HPSS users toselect 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 maximumfile size, transfer rate, access frequency, latency, and valid read or write operations. A file resides ina particular COS and the COS is selected when the file is created. Underlying a COS is a storagehierarchy that describes how data for files in that class are to be stored in the HPSS system. A COScan 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 oftape virtual volumes. HPSS supports grouping of files only on tape volumes. In addition, familiescan only be specified by associating a family with a fileset, and creating the file in the fileset. Whena file is migrated from disk to tape, it is migrated to a tape virtual volume assigned to the familyassociated with the file. If no family is associated with the file, the file is migrated to the nextavailable tape not associated with a family (actually to a tape associated with family zero). If the fileis associated with a family and no tape VV is available for writing in the family, a blank tape isreassigned from family zero to the file’s family. The family affiliation is preserved when tapes arerepacked.The relationship between storage class, storage hierarchy, and COS is shown in Figure 2-2.
