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Chapter 2 HPSS Planning
84 September 2002 HPSS Installation Guide
Release 4.5, Revision 2
laterally to another volume in the same storage class. Tape file migration with purge
avoids moving read active files at all. If a file is read inactive, all three algorithms migrate
it down the hierarchy. The purpose of this field is to avoid removing the higher level copy
of a file which is likely to be staged again.
• The Last Update Interval is used by all of the tape migration algorithms to determine if a
file is actively being written. A file which has been written more recently than the number
of minutes specified in this field is considered active. If a file is write active, tape volume
migration moves it laterally instead of migrating it downwards. Tape file migration
avoids any operations on write active files. The purpose of this field is to avoid performing
a migration on a file which is likely to be rewritten and thus need to be migrated yet again.
• The Free Space Target parameter controls the total number of free virtual volumes a site
intends to maintain. It is suggested that this field always be set to 100%.
• The Request Count field is used by the tape file migration algorithms only to determine
the number of concurrent migration threads used by each migration run. Each migration
thread operates on a separate source volume, so, this parameter determines the number of
volumes which are migrated at once.
• The Runtime Interval is used by all of the tape migration algorithms to control how often
migration runs for a given storage class. In addition, the administrator can force a
migration run to start via SSM.
• The Migrate Volumes flag selects the tape volume migration algorithm.
• The Migrate Volumes and Whole Files flag modifies the tape volume migration algorithm
to avoid having the storage segments of a single file scattered over different tapes. When
this field is set, if a file on a virtual volume is selected to be migrated to the next level, any
parts of this file that are on different virtual volumes are also migrated even if they would
ordinarily not meet the criteria for being migrated. This tends to pack all the storage
segments for a given file on the same virtual volume.
• The Migrate Files flag selects the tape file migration algorithm.
• The Migrate Files and Purge modifies the tape file migration algorithm such that the
higher level copy is removed once the lower level copy is created. Only one copy of each
file is maintained.
2.8.2 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 should be defined for all disk
storage classes that support migration. 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 storage class by periodically extracting this
information from the HPSS Storage Servers. Based upon parameters in the purge policy, a purge
run will be started when appropriate. The administrator can also force the start of a purge run via
SSM.
The disk purge is controlled by several parameters: