Migration/Purge Server - IBM High Performance Storage System HPSS Installation Manual

The Core Server is designed to scale up its ability to manage tapes as the number of tapes increases.
As long as sufficient memory and CPU capacity exist, threads can be added to the server to increase
its throughput. Additional subsystems can also be added to a system, increasing concurrency even
further.
Note that the number of tape drives the server manages has much more to do with the throughput of
the server than the number of tape volumes the server manages. If the number of tape drives in the
system needs to increase to meet workload demands, adding a new subsystem and redistributing the
drives may be the best way to deal with the increased workload.

3.7.2. Migration/Purge Server

The Migration/Purge Server (MPS) reports storage class usage statistics and manages the amount of
free space available in storage classes by performing periodic migration and purge runs on the storage
classes. Migration copies data from the storage class on which it runs to one or more lower levels in
the storage hierarchy. Once data has been migrated, a subsequent purge run will delete the data from
the migrated storage class, if so configured. Migration is a prerequisite for purge, and MPS will never
purge data which has not previously been migrated. It is possible, but not desirable, to assign only a
migration policy and no purge policy to a disk storage class; however, this will result in data being
copied (migrated) but never deleted (purged). It is important to recognize that migration and purge
policies determine when data is migrated from a storage class and when the data is purged from that
storage class; however, the number of copies and the location of those copies is determined by the
storage hierarchy definition. The MPS uses the Core Server to both perform data movement between
hierarchy levels and gather statistics. As such, the Core Server must be running in order for the MPS
to complete its duties.
The MPS can only exist within a storage subsystem and a subsystem may be configured with no more
than one MPS. Storage hierarchies are global across all storage subsystems within an HPSS system,
but a given hierarchy may or may not be enabled within a given subsystem (a hierarchy is enabled
within a subsystem by configuring the subsystem to enable one or more classes of service which
reference that hierarchy). Note that a storage class may not be selectively migrated and purged in
different subsystems. If the hierarchy contains storage classes which require migration and purge,
then an MPS must be configured to run against those storage classes in the subsystem. This MPS will
manage migration and purge operations on only those storage resources within its assigned
subsystem. Thus, for an HPSS system with multiple storage subsystems, there may be multiple MPSs,
each operating on the resources within a particular subsystem.
Migration and purge operate differently on disk and tape storage classes. Disk migration and disk
purge are configured on a disk storage class by associating a migration policy and a purge policy with
that storage class. For tape storage classes, the migration and purge operations are combined, and are
collectively referred to as tape migration. Tape migration is enabled by associating a migration policy
with a tape storage class. Purge policies are not needed or supported on tape storage classes.
Once migration and purge policies are configured for a storage class (and the MPS is restarted), the
MPS will begin scheduling migration and purge runs for that storage class. Migration on both disk
and tape is run periodically according to the runtime interval configured in the migration policy. Disk
purge runs are not scheduled periodically, but rather are started when the percentage of space used in
the storage class reaches the threshold configured in the purge policy for that storage class. It is
critical that the hierarchies to which a storage class belongs be configured with proper migration
targets in order for migration and purge to perform as expected.
The purpose of disk purge is to maintain a given amount of free space in a disk storage class by
removing data of which copies exist at lower levels in the hierarchy. The order in which purge
records are sorted, which determines the order in which files are purged, may be configured on the
purge policy. It should be noted that all of the options except Record Create Time require additional
HPSS Installation Guide July 2008
Release 6.2 (Revision 2.0) 83