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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
Storage Servers, MPS copies data to lower levels in the hierarchy (migration), removes data
from the current level oncecopies have been made (purge), ormoves data between volumes
at the same level (lateral move). Based on the hierarchy configuration, MPS can be directed
to create duplicate copies of data when it is being migrated from disk or tape. This is 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. Disk purge should always be run along with disk migration. The designation disk
or taperefers to the type of storage class that migrationis running against. See Section 2.6.5:
Migration/Purge Server on page 65 for a more complete discussion of the different types of
migration.
Disk Migration/Purge:
The purpose 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
subsequently removes 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
storage class to a lower level in the hierarchy. It is 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:
The purpose 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
(caused by users overwriting or deleting files). The remaining segments on these volumes
are either migrated downwards to the next level in the hierarchy, or are moved laterally to
another tape volume at the same level. This results in empty tape volumes which may then
be reclaimed. Note that there is no purge component to tape volume migration. All of the
operations use a move instead of a copy semantic.
MPS runs 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 ofphysical resources intostriped virtual volumes to allow