Chapter 1 HPSS Basics
HPSS Installation Guide September 2002 27
Release 4.5, Revision 2
•Migration/Purge Server (MPS). The MPS allows the local site to implement its storage
management policies by managing the placement of data on HPSS storage media using
site-defined migration and purge policies. By making appropriate calls to the Bitfile and
StorageServers, MPS copies data to lower levels in the hierarchy (migration), removes data
fromthe current level once copies have been made (purge), or moves data between volumes
atthe same level (lateral move). Basedon the hierarchy configuration, MPS can be directed
tocreate duplicate copies of data when it is being migrated from disk or tape. Thisis done
by copying the data to one or more lower levels in the storage hierarchy.
There are three types of migration:disk migration,tape file migration, and tape volume
migration.Diskpurge should always be run along with disk migration. Thedesignation disk
ortape refers to the type of storage class that migration is running against. SeeSection2.6.5:
Migration/Purge Server on page 65 for a more complete discussion of the different types of
migration.
Disk Migration/Purge:
Thepurpose of disk migration is to make one or more copies of disk files to lower levels in
the hierarchy. The number of copies depends on the configuration of the hierarchy. For
disk, migration and purge are separate operations. Any disk storage class which is
configured for migration should be configured for purge as well. Once a file has been
migrated (copied) downwards in the hierarchy, it becomes eligible for purge, which
subsequentlyremoves the file from the current level and allows the disk space to be reused.
Tape File Migration:
The purpose of tape file migration is to make a single, additional copy of files in a tape
storageclass to a lower level in the hierarchy. Itis also possible to move files downwards
instead of copying them. In this case there is no duplicate copy maintained.
That there is no separate purge component to tape file migration, and tape volumes will
require manual repack and reclaim operations to be performed by the admin.
Tape Volume Migration:
Thepurpose of tape volume migration is to free tape volumes for reuse. Tape volumes are
selected based on being in the EOM map state and containing the most unused space
(causedby users overwriting or deleting files). Theremaining segments on these volumes
areeither migrated downwards to the next level in the hierarchy, or are moved laterally to
anothertape volume at the same level. Thisresultsin empty tape volumes which may then
bereclaimed. Note that there is no purge component to tape volume migration. Allof the
operations use amove instead of a copy semantic.
MPSruns migration on each storage class periodically using the time interval specified in
the migration policy for that class. See Section 1.3.7: HPSS Policy Modules on page 35 for
details on migration and purge policies. In addition, migration runs can be started
automatically when the warning or critical space thresholds for the storage class are
exceeded. Purge runs are started automatically on each storage class when the free space
in that class falls below the percentage specified in the purge policy.
•Storage Server (SS). The Storage Servers provide a hierarchy of storage objects: storage
segments, virtual volumes, and physical volumes. The Storage Servers translates storage
segment references into virtual volume references and then into physical volume
references,handles the mapping of physical resources into striped virtual volumes to allow