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Disk migration is controlled by several parameters. By default, these parameters are the same across
all subsystems. However, subsystem-specific policies may be created which override all of these
values. For a list of these parameters, refer to Section 6.4.2.2: Disk Migration Policy Configuration
in the HPSS Management Guide.
3.9.1.2. Migration Policy for Tape
There are two tape migration algorithms: tape file migration and tape volume migration. The
algorithm which MPS applies to a given tape storage class is selected in the migration policy for that
storage class.
The purpose of tape file migration is to make a second copy of files written on tape. This algorithm is
similar to disk migration, but only a single additional copy is possible. It is also possible to configure
tape file migration such that files are moved downwards in the hierarchy without keeping a second
copy.
It is possible for tape file migration to make a second copy of files written on tape. The algorithm is
similar to disk file migration, but only a single additional copy is possible. It is also possible to
configure tape file migration such that files are moved downwards in the hierarchy without keeping a
second copy.
The purpose of tape volume migration is to empty tape virtual volumes that have become full
(reached EOM) and have significant unused space in them. Unused space on a tape volume is
generated when files on that tape are deleted or overwritten. Since data can only be recorded on tapes
sequentially, vacated recording space on tapes can be reclaimed only by moving all remaining files to
other tapes.
Tape volume migration attempts to empty tapes by moving data off of the tapes to other volumes.
When a tape becomes empty, it is a candidate for reuse. The reclaim utility program resets the state
of the empty tape volumes so that they can be reused. The reclaim utility can be run from SSM, but it
should generally be set up to run on a periodic basis via the cron facility. For more information on
reclaim, see Section 8.1.5: Reclaiming HPSS Tape Virtual Volumes of the HPSS Management Guide
and the reclaim manual page.
The repack utility can also be used to create empty tapes in a storage class. The administrator should
determine whether a tape should be repacked based on the number of holes (due to file overwrite or
deletion) on the tape. If a tape storage class is at the bottom of a hierarchy, repack and reclaim must
be run periodically to reclaim wasted space. For more information on repack, see Section 8.1.4:
Repacking Tape Virtual Volumes of the HPSS Management Guide and the repack manual page.
The migration policy parameters which apply to the different tape migration algorithms in detail in
Section 6.4.2.3: Tape Migration Policy Configuration in the HPSS Management Guide.
3.9.2. Purge Policy
The purge policy allows the MPS to remove the bitfiles from disk after the bitfiles have been
migrated to a lower level of storage in the hierarchy. A purge policy cannot be defined for a tape
storage class or a disk storage class which does not support migration. Sites may or may not wish to
define a purge policy for all disk storage classes that support migration. Purging from tapes is
controlled by the "Migrate Files and Purge" flag of the tape migration policy; there is no separate
purge policy for tape storage classes.
The specification of the purge policy in the storage class configuration enables the MPS to do the
disk purging according to the purge policy for that particular storage class. Purge is run for a storage
class on a demand basis. The MPS maintains current information on total space and free space in a
HPSS Installation Guide July 2008
Release 6.2 (Revision 2.0) 95