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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 long as sufficient memory and CPU capacity exist, threads can be added to the server
to increase its throughput. Additional StorageSubsystems can also be added to asystem, increasing
concurrency even further.
Note that the number of tape units the server manages has much more to do with the throughput
of the server than the number of tapes the server manages. If the number of tape units in the system
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
subsystem which is configured to make use of a storage hierarchy which requires the migration and
purge operations must be configured with one and only one MPS within that subsystem. The
definition of storage hierarchies is global across all storage subsystems within an HPSS 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 must be assigned to the storage classes in that hierarchy for that subsystem. This is done
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. This MPS will manage migration and purge operations on only those storage resources
within its assigned subsystem. Hence, in an HPSS system with multiple storage subsystems, 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
subsystem by performing periodic migration and purge runs onthat storage class.Migration copies
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
previously been migrated. It is important to recognize that migration and purge policies determine
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
hierarchy definition. Note that this is a major difference between release 4.2+ versions of 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 the storage class definition is global across all storage subsystems, a storage class may not be
selectively migrated and purged in different subsystems. Additionally, migration and purge
operate differently on disk and tape storage classes. Disk migration and disk purge are configured on
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.
Tape migration is enabled by associating a migration policy with a tape storage class. Purge policies
are not needed or supported on tape storage classes.
Once migration and purge are configured for a storage class (and MPS is restarted), MPS will begin
scheduling migration and purge runs for that storage class. Migration on both disk and tape is 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