Hub/Switch Installation Guide

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,MPScopies datato lowerlevels in thehierarchy (migration),removesdata

fromthecurrentleveloncecopieshavebeenmade(purge),ormovesdatabetweenvolumes

at thesame level (lateral move). Based on thehierarchy conﬁguration,MPS can be directed

to createduplicate copiesof datawhenitisbeingmigratedfrom 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.Diskpurgeshould alwaysberunalongwithdiskmigration. Thedesignationdisk

ortapereferstothetypeofstorageclassthatmigrationisrunningagainst. SeeSection2.6.5:

Migration/Purge Server on page 65 for a more complete discussion of the different types of

migration.

Disk Migration/Purge:

The purpose of disk migrationis tomake oneormorecopies ofdiskﬁlestolower 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

subsequentlyremovesthe ﬁlefromthe currentleveland allowsthe diskspacetobereused.

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 purposeof tape volume migration isto free tape volumesfor reuse. Tape volumes are

selected based on being in the EOM map state and containing the most unused space

(caused by usersoverwritingordeletingﬁles). The remaining segments on thesevolumes

areeither migrated downwardsto the nextlevel in the hierarchy, oraremoved laterally to

anothertapevolumeatthesamelevel. Thisresultsinemptytapevolumeswhichmaythen

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,handlesthemappingofphysicalresourcesintostripedvirtualvolumestoallow