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Chapter 2 HPSS Planning
82 September 2002 HPSS Installation Guide
Release 4.5, Revision 2
2.8.1 Migration Policy
The migration policy provides the capability for HPSS to copy (migrate) data from one level in a
hierarchy to one or more lower levels. The migration policy defines the amount of data and the
conditions under which it is migrated, but the number of copies and the location of those copies is
determined by the storage hierarchy definition. The site administrator will need to monitor the
usage of the storage classes being migrated and adjust both the migration and purge policies to
obtain the desired results.
2.8.1.1 Migration Policy for Disk
Disk migration in HPSS copies (migrates) files from a disk storage class to one or more lower levels
in the storage hierarchy. Removing or purging of the files from disk storage class is controlled by
the purge policy. The migration and purge policies work in conjunction to maintain sufficient
storage space in the disk storage class.
When data is copied from the disk, the copied data will be marked purgeable but will not be
deleted. Data is deleted by running purge on the storage class. If duplicate copies are created, the
copied data is not marked purgeable until all copies have been successfully created. The migration
policy and purge policy associated with a disk storage class must be set up to provide sufficient free
space to deal with demand for storage. This involves setting the parameters in the migration policy
to migrate a sufficient amount of files and setting the purge policy to reclaim enough of this disk
space to provide the free space desired for users of the disk storage class.
Disk migration is controlled by several parameters:
• The Last Update Interval isused to prevent files thathave been writtenrecently from being
migrated. Files that have been updated within this interval are not candidates for
migration. Setting this value too high may limit how much data can be migrated and thus
marked purgeable. This may prevent purge from purging enough free space to meet user
demands. Settingthis value too low could cause the same file to be migratedmultiple times
while it is being updated. The setting of this parameter should be driven by the amount of
disk spacein the storage class andhow much new data is written tothe storage class within
a given time period.
• The Free Space Target controls the number of bytes to be copied by a migration run. The
value of this parameter is important in association with the purge policy. The amount of
data that is copied is potentially purgeable when the next purge on this storage class is run.
This value must be set at a sufficient level so that enough purgeable space is created for the
purge to meet the free space demands for users of this storage class. If this value is set to
other than 100%, the time at which copies are made of disk files will be unpredictable.
• The Runtime Interval is used to control how often migration will run for this storage class.
The administrator can also force a migration run to start via SSM. The value of this
parameter is determined by the amount of disk space and the utilization of that disk space
by users of the storage class. If the amount of disk space is relatively small and heavily
used, the Runtime Interval may have to be set lower to meet the user requirements for free
space in this storage class.
• The Request Count is used to specify the number of parallel migration threads which are
used for each destination level (i.e. copy) to be migrated.