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Chapter 2 HPSS Planning
102 September 2002 HPSS Installation Guide
Release 4.5, Revision 2
and its associated attributes. For detailed descriptions of each attribute associated with a storage
hierarchy, see Section 6.7.2: Configure the Storage Hierarchies (page 315). The following is a list of
rules 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 Storage
Server.
Rule 3: Parts or all of a file may appear at multiple levels in a storage hierarchy. If data for a file does
appear 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 creating
duplicate 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. If
data is to be migrated from this disk to a tape storage class, the storage segment size as specified by
the Minimum Storage Segment Size parameter in the storage class definition should meet one of
the following conditions. These rules are associated with the internal migration process. If not
adhered to, can result in excessive storage segment creations.
• Storage segment size on disk is an integral multiple of the stripe length on tape. If this
option 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 Service
Each HPSS file belongs to a single Class of Service (COS) that is selected when the file is created. It
is selected via Class of Service Hints information passed to the Bitfile Server when the bitfile is
created. If using the Client API, the application program has full access to this hints information. If
using NFS, the COS is the same for each file in a fileset. If no class of services is assigned to the
fileset, then the COS is determined by the COS ID that is specified by the NFS Daemon’s
configuration parameters. For FTP, there is a quote command to set the desired COS ID. A pput
request in PFTP automatically selects a COS based on file size by using the COS Hints unless the
user explicitly selects the COS. The SSM provides operational capabilities to define classes of
service. A COS is identified by a COS ID and its associated attributes. For detailed descriptions of
each 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 flag
is 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 of
service.