IBM Hub/Switch Installation Manual

Also see for High Performance Storage System HPSS: Management guide Installation guide

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Chapter 2 HPSS PlanningHPSS Installation Guide September 2002 65Release 4.5, Revision 2The Tape Storage Server is designed to scale up its ability to manage tapes as the number of tapesincreases. As long as sufficient memory and CPU capacity exist, threads can be added to the serverto increase its throughput. Additional Storage Subsystems can also be added to a system, increasingconcurrency even further.Note that the number of tape units the server manages has much more to do with the throughputof the server than the number of tapes the server manages. If the number of tape units in the systemincreases, adding a new Tape Storage Server to the system may be the best way to deal with theincreased load.2.6.5 Migration/Purge ServerThe Migration/Purge Server (MPS) can only exist within a storage subsystem. Any storagesubsystem which is configured to make use of a storage hierarchy which requires the migration andpurge operations must be configured with one and only one MPS within that subsystem. Thedefinition of storage hierarchies is global across all storage subsystems within an HPSS system, buta given hierarchy may or may not be enabled within a given subsystem. A hierarchy is enabledwithin a subsystem by using the storage subsystem configuration to enable one or more classes ofservice which reference that hierarchy. If a hierarchy is enabled within a subsystem, storageresources must be assigned to the storage classes in that hierarchy for that subsystem. This is doneby creating resources for the Storage Servers in the given subsystem. If the hierarchy containsstorage classes which require migration and purge, then an MPS must be configured in thesubsystem. This MPS will manage migration and purge operations on only those storage resourceswithin its assigned subsystem. Hence, in an HPSS system with multiple storage subsystems, theremay be multiple MPSs, each operating on the resources within a particular subsystem.MPS manages the amount of free space available in a storage class within its assigned storagesubsystem by performing periodic migration and purge runs on that storage class. Migration copiesdata from the storage class on which it runs to one or more lower levels in the storage hierarchy.Once data has been migrated, a subsequent purge run will delete the data from the migratedstorage class. Migration is a prerequisite for purge, and MPS will never purge data which has notpreviously been migrated. It is important to recognize that migration and purge policies determinewhen data is copied from a storage class and then when the data is deleted from that storage class;however, the number of copies and the location of those copies is determined solely by the storagehierarchy definition. Note that this is a major difference between release 4.2+ versions of the HPSSsystem and all previous releases.Migration and purge must be configured for each storage class on which they are desired to run.Since the storage class definition is global across all storage subsystems, a storage class may not beselectively migrated and purged in different subsystems. Additionally, migration and purgeoperate differently on disk and tape storage classes. Disk migration and disk purge are configured ona disk storage class by associating a migration policy and a purge policy with that storage class. It ispossible, but not desirable, to assign only a migration policy and no purge policy to a disk storageclass; however, this will result in data being copied but never deleted. For tape storage classes, themigration and purge operations are combined, and are collectively referred to as tape migration.Tape migration is enabled by associating a migration policy with a tape storage class. Purge policiesare not needed or supported on tape storage classes.Once migration and purge are configured for a storage class (and MPS is restarted), MPS will beginscheduling migration and purge runs for that storage class. Migration on both disk and tape is runperiodically according to the runtime interval configured in the migration policy. Disk purge runsare not scheduled periodically, but rather are started when the percentage of space used in the