This section defines key HPSS storage concepts which have a significant impact on the usability of HPSS. Configuration of the HPSS storage objects and policies is the responsibility of your HPSS administrator.
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 characteristics such 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 class 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 HPSS.
For the FTP and PFTP interfaces, the COS ID may be explicitly specified by using the " site setcos " command. If not specified, a default COS is used. For NFS, all files are created in the same COS. This COS is defined by your system administrator. Otherwise, the size of the file is used as hints for COS selection. Contact your HPSS administrator to determine the COSs which have been defined. The " lshpss -cos " command may also be used to list the defined COSs. Refer to Chapter 5 for information on the " lshpss " command.
Also, PFTP provides a feature to automatically store the local file size in the minimum and maximum file size fields of the COS. This feature is also provided for FTP clients which support the ALLO command. This allows the COS selection to be made according to file size. The HPSS administrator should ensure that COS definitions contain proper minimum and maximum file sizes in order for PFTP (FTP clients which support ALLO) to optimize storage utilization when tranferring files to HPSS. Note: If the COS ID is explicitly set by using the " site setcos " command, that COS will be used regardless of file size.
A COS is implemented by a Storage Hierarchy of one to many Storage Classes. Storage Hierarchies and Storage Classes are not directly visible to the user, but are described below since they map to COS..
An HPSS Storage Class is used to group storage media together to provide storage with specific characteristics for HPSS data. The attributes associated with a storage class are both physical and logical. Physical media in HPSS are called physical volumes. Physical characteristics associated with physical volumes are the media type, block size, the estimated amount of space on volumes in this class, and how often to write tape marks on the volume (for tape only). Physical media are organized into logical virtual volumes. This allows striping of physical volumes. Some of the logical attributes associated with the storage class are virtual volume block size, stripe width, data transfer rate, latency associated with devices supporting the physical media in this class, and storage segment size (disk only). In addition, the storage class has attributes that associate it with a particular migration policy and purge policy to help in managing the total space in the storage class.
An HPSS storage hierarchy consists of multiple levels of storage with each level representing a different storage media (i.e., a 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 request. 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 on the migration policy and staging operations. Multiple copies of a file may also be specified in the migration policy. 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.
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 for files by associating a fileset with a family, and creating the files in that 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 no tape virtual volume is associated with the family, a blank tape is reassigned from family zero to the file's family. The family affiliation is preserved when tapes are repacked. Configuring file families is a System Administrator function.