Chapter 2 HPSS Planning102 September 2002 HPSS Installation GuideRelease 4.5, Revision 2and its associated attributes. For detailed descriptions of each attribute associated with a storagehierarchy, see Section 6.7.2: Configure the Storage Hierarchies (page 315). The following is a list ofrules and guidelines for creating and managing storage hierarchies.Rule 1: All writes initiated by clients are directed to the highest level (level 0) in the hierarchy.Rule 2: The data of a file at a storage class level in a hierarchy is associated with a single StorageServer.Rule 3: Parts or all of a file may appear at multiple levels in a storage hierarchy. If data for a file doesappear at multiple levels of the hierarchy, the data at the higher level is always the more recent data.Rule 4: Migration of data does not skip levels in the hierarchy, except in the special case of creatingduplicate copies when doing disk migration.Rule 5: The client stage command can only stage data to the top level (level 0) in the hierarchy.Rule 6: A given storage class can only occur once in the same hierarchy.Guideline: Care must be taken when selecting the storage segment size for a disk storage class. Ifdata is to be migrated from this disk to a tape storage class, the storage segment size as specified bythe Minimum Storage Segment Size parameter in the storage class definition should meet one ofthe following conditions. These rules are associated with the internal migration process. If notadhered to, can result in excessive storage segment creations.• Storage segment size on disk is an integral multiple of the stripe length on tape. If thisoption is selected, normally these values would be set equal.• Stripe length on tape is an integral multiple of the storage segment size on disk. In this case,the multiple must be less than or equal to 16.2.9.3 Class of ServiceEach HPSS file belongs to a single Class of Service (COS) that is selected when the file is created. Itis selected via Class of Service Hints information passed to the Bitfile Server when the bitfile iscreated. If using the Client API, the application program has full access to this hints information. Ifusing NFS, the COS is the same for each file in a fileset. If no class of services is assigned to thefileset, then the COS is determined by the COS ID that is specified by the NFS Daemon’sconfiguration parameters. For FTP, there is a quote command to set the desired COS ID. A pputrequest in PFTP automatically selects a COS based on file size by using the COS Hints unless theuser explicitly selects the COS. The SSM provides operational capabilities to define classes ofservice. A COS is identified by a COS ID and its associated attributes. For detailed descriptions ofeach attribute associated with a class of service, see Section 5.5.3.The Force Selection flag can be set in the COS definition to eliminate automatic selection. If this flagis set, the designated COS can only be selected by asking for the COS by ID or Name.The paragraphs that follow give guidelines and explanations for creating and managing classes ofservice.
