IBM Hub/Switch Installation Manual

Also see for High Performance Storage System HPSS: Management guide Installation guide

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Chapter 2 HPSS PlanningHPSS Installation Guide September 2002 83Release 4.5, Revision 2• The Migrate At Warning Threshold option causes MPS to begin a migration runimmediately when the storage class warning threshold is reached regardless of when theRuntime Interval is due to expire. This option allows MPS to begin migrationautomatically when it senses that a storage space crisis may be approaching.• The Migrate At Critical Threshold option works the same as the Migrate At WarningThreshold option except that this flag applies to the critical threshold. Note that if thecritical threshold is set to a higher percentage than the warning threshold (as it should befor disk storage classes), then the critical threshold being exceeded implies that thewarning threshold has also been exceeded. If Migrate At Warning Threshold is set, thenMigrate At Critical Threshold does not need to be set.2.8.1.2 Migration Policy for TapeThere are 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 policyfor that storage class.The purpose of tape file migration is to make a second copy of files written on tape. This algorithmis similar to disk migration, but only a single additional copy is possible. It is also possible toconfigure tape file migration such that files are moved downwards in the hierarchy withoutkeeping 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 generatedwhen files on that tape are deleted or overwritten. This happens because tape is a sequential accessmedia and new data is always written at the end of the tape. When data is deleted from the tapevolume, the space associated with the data cannot be reused. The only way to reuse space on a tapeis to copy all of the valid data off of the tape and then reclaim the empty volume.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. A special utility called reclaim resets thestate of the empty tape volumes so that they can be reused. The reclaim utility can be run fromSSM, but it should generally be set up to run on a periodic basis via the cron facility. For moreinformation on reclaim, see Section 3.8: Reclaiming HPSS Tape Virtual Volumes on page 76 of theHPSS Management Guide and Section 12.2.47: reclaim — HPSS Volume Reclaim Utility on page 438 ofthe HPSS Management Guide.The repack utility can also be used to create empty tapes in a storage class. The administratorshould determine whether a tape should be repacked based on the number of holes (due to fileoverwrite or deletion) on the tape. If a tape storage class is at the bottom of a hierarchy, repack andreclaim must be run periodically to reclaim wasted space. For more information on repack, seeSection 3.7: 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 the HPSS Management Guide.The migration policy parameters which apply to the different tape migration algorithms aredescribed below. Parameters which only apply to disk migration are not described.• The Last Read Interval parameter is used by both tape volume migration algorithms aswell as tape file migration with purge to determine if a file is actively being read or isinactive. A file which has been read more recently than the number of minutes specified inthis field is considered active. If a file is read active, tape volume migration moves it