Hub/Switch Installation Guide

Chapter 2 HPSS Planning

HPSS Installation Guide September 2002 83

Release 4.5, Revision 2

• The Migrate 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 the Migrate 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

warning threshold has also been exceeded. If Migrate 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: tapeﬁle migrationand tapevolumemigration.

The algorithm which MPS applies to a given tape storage class is selected in the migration policy

for that storage class.

Thepurposeof tapeﬁle migrationis tomake asecondcopy ofﬁles writtenontape. 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 onthat tapearedeletedoroverwritten. Thishappens becausetape isasequentialaccess

media and new data is always written at the end of the tape. When data is deleted from the tape

volume,thespaceassociatedwith thedatacannotbereused. The onlywaytoreusespaceon atape

is to copy all of the valid data off of the tape and then reclaim the empty volume.

Tape volumemigration attemptsto empty tapes by moving dataoff of the tapesto othervolumes.

When a tape becomes empty, it is a candidate for reuse. A special utility called reclaim 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 3.8: Reclaiming HPSS Tape Virtual Volumes on page 76 of the

HPSS Management Guideand Section 12.2.47: reclaim — HPSSVolume Reclaim Utility onpage 438 of

the HPSS 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

overwriteordeletion) onthe tape. Ifa tapestorage classisat thebottom ofahierarchy,repackand

reclaim must be run periodically to reclaim wasted space. For more information on repack, see

Section 3.7:RepackingHPSSVolumes onpage 74of theHPSS Management Guide andSection12.2.51:

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 are

described below. Parameters which only apply to disk migration are not described.

• The Last 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 ﬁlewhichhas beenreadmorerecentlythanthenumber ofminutesspeciﬁed in

this ﬁeld is considered active. If a ﬁle is read active, tape volume migration moves it