Chapter 2 HPSS Planning
HPSS Installation Guide September 2002 83
Release 4.5, Revision 2
• TheMigrate At Warning Threshold option causes MPS to begin a migration run
immediately when the storage class warning threshold is reached regardless of when the
Runtime Interval is due to expire. This option allows MPS to begin migration
automatically when it senses that a storage space crisis may be approaching.
• The Migrate At Critical Threshold option works the same as theMigrate At Warning
Threshold option except that this flag applies to the critical threshold. Note that if the
critical threshold is set to a higher percentage than the warning threshold (as it should be
for disk storage classes), then the critical threshold being exceeded implies that the
warningthreshold has also been exceeded. IfMigrate At Warning Threshold is set, then
Migrate At Critical Threshold does not need to be set.
2.8.1.2 Migration Policy for Tape
Thereare two different 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.
Thepurpose of tape file migration is to make a second copy of files written on tape. Thisalgorithm
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.
The purpose of tape volume migration is to free up tape virtual volumes that have become full
(EOM) and have significant unused space on them. Unused space on a tape volume is generated
whenfiles on that tape are deleted or overwritten. Thishappens because tape is a sequential access
media and new data is always written at the end of the tape. When data is deleted from the tape
volume,the space associated with the data cannot be reused. Theonlyway to reuse space on a tape
is to copy all of the valid data off of the tape and then reclaim the empty volume.
Tapevolume 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. A special utility calledreclaim resets the
state of the empty tape volumes so that they can be reused. Thereclaim utility can be run from
SSM, but it should generally be set up to run on a periodic basis via thecron facility. For more
information onreclaim, see Section 3.8: Reclaiming HPSS Tape Virtual Volumes on page 76 of the
HPSSManagement Guide and Section 12.2.47: reclaim — HPSS Volume Reclaim Utility on page 438 of
theHPSS Management Guide.
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
overwriteor deletion) on the tape. Ifa tape storage class is at the bottom of a hierarchy,repack and
reclaim must be run periodically to reclaim wasted space. For more information onrepack, see
Section3.7: Repacking HPSS Volumes on page 74 of the HPSS Management Guide and Section 12.2.51:
repack — HPSS Volume Repack Utility on page 452 of theHPSS Management Guide.
The migration policy parameters which apply to the different tape migration algorithms are
described below. Parameters which only apply to disk migration are not described.
• TheLast Read Interval parameter is used by both tape volume migration algorithms as
well as tape file migration with purge to determine if a file is actively being read or is
inactive. A file which has been read more recently than the number of minutes specified in
this field is considered active. If a file is read active, tape volume migration moves it