Hub/Switch Installation Guide

Chapter 2 HPSS Planning
HPSS Installation Guide September 2002 65
Release 4.5, Revision 2
The Tape Storage Server is designed to scale up its ability to manage tapes as the number of tapes
increases.As longas sufficient memoryandCPUcapacityexist,threads can be added totheserver
toincreaseitsthroughput.AdditionalStorageSubsystemscanalsobeaddedtoasystem,increasing
concurrency even further.
Note that the number of tape units the server manages has much more to do with the throughput
ofthe serverthanthenumberoftapesthe servermanages. Ifthenumberoftapeunitsinthesystem
increases, adding a new Tape Storage Server to the system may be the best way to deal with the
increased load.
2.6.5 Migration/Purge Server
The Migration/Purge Server (MPS) can only exist within a storage subsystem. Any storage
subsystemwhichis configuredto makeuseof astoragehierarchywhich requiresthemigrationand
purge operations must be configured with one and only one MPS within that subsystem. The
definitionofstorage hierarchiesis globalacrossallstorage subsystemswithinanHPSS system,but
a given hierarchy may or may not be enabled within a given subsystem. A hierarchy is enabled
within a subsystem by using the storage subsystem configuration to enable one or more classes of
service which reference that hierarchy. If a hierarchy is enabled within a subsystem, storage
resources mustbe assigned tothe storage classesin that hierarchy for thatsubsystem. This isdone
by creating resources for the Storage Servers in the given subsystem. If the hierarchy contains
storage classes which require migration and purge, then an MPS must be configured in the
subsystem.ThisMPS will managemigration andpurgeoperations on onlythose storageresources
within its assigned subsystem. Hence, in an HPSSsystem withmultiple storagesubsystems, there
may 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 storage
subsystembyperformingperiodicmigrationandpurgerunsonthatstorageclass.Migrationcopies
data 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 migrated
storage class. Migration is a prerequisite for purge, and MPS will never purge data which has not
previouslybeenmigrated.It isimportant torecognizethatmigrationand purgepoliciesdetermine
when 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 storage
hierarchydefinition. Note that this isa major differencebetween release 4.2+ versionsof the HPSS
system and all previous releases.
Migration and purge must be configured for each storage class on which they are desired to run.
Since thestorage class definition isglobal acrossall storage subsystems, astorage class may notbe
selectively migrated and purged in different subsystems. Additionally, migration and purge
operatedifferentlyondisk andtape storage classes. Diskmigrationand diskpurgeare configuredon
a disk storage class by associating a migration policy and a purge policy with that storage class. It is
possible, but not desirable, to assign only a migration policy and no purge policy to a disk storage
class; however, this will result in data being copied but never deleted. For tape storage classes, the
migration and purge operations are combined, and are collectively referred to as tape migration.
Tapemigrationisenabledbyassociatingamigrationpolicywithatapestorageclass.Purgepolicies
are not needed or supported on tape storage classes.
Oncemigration andpurgeareconfiguredfor astorageclass (andMPS isrestarted),MPSwill begin
scheduling migration andpurge runs for thatstorage class.Migration on both diskand tapeis run
periodically according to the runtime interval configured in the migration policy. Disk purge runs
are not scheduled periodically, but rather are started when the percentage of space used in the