Chapter 2 HPSS Planning
HPSS Installation Guide September 2002 67
Release 4.5, Revision 2
Thereare two different tape migration algorithms, tape volume migration and tape file migration. The
algorithm which is applied to a tape storage class is selected in the migration policy for that class.
The purpose of tape volume migration is to move data stored in a tape storage class either
downward to the next level of the storage hierarchy (migration) or to another tape volume within
thesame storage class (lateral move) in order to empty tape volumes and allow them to be reclaimed.
Unlike disk migration, the data is purged from the source volume as soon as it is copied. Tape
volume migration operates on storage segments rather than files. A file may contain one or more
segments. In order for a segment to be a candidate for tape volume migration it must reside on a
virtual volume whose storage map is in the EOM state. Tape volume migration functions by
selecting EOM volumes and moving or migrating the segments off of these volumes. When a
volume is selected for tape volume migration, MPS repeatedly processes lists of up to 3200
segmentson that volume until the volume is empty. Onceall of the segments have been removed
froma volume, that volume automatically moves into the EMPTY state and may be reclaimed for
reuse. MPS continues this process until either the percentage of volumes specified in the migration
policy are emptied or no more EOM volumes can be found. Segments on an EOM volume are
evaluated for tape volume migration based on the values in the migration policy for that storage
class. If a segment has been inactive for a sufficient length of time it will be migrated. If a segment
has been active within the configured amount of time, or if any other segment in the selected
segment'sfile has been active, the selected segment will be moved laterally. TheMigrate Volumes
andWhole Files option in the migration policy allows all of the segments belonging to a file to be
migratedtogether, including those segments which reside on other, potentially non-EOM volumes
than the EOM volume which is being processed. This option tends to keep all of the segments
belongingto a given file at the same level in the hierarchy. If a segment is selected for migration by
MPS, then all other segments belonging to the same file, regardless of their location, will be
migratedduring the same migration run. If any of the segments in the file are active then none of
them,including the segment on the selected EOM volume, will be allowed to migrate. Rather,the
selectedsegment will be moved laterally and none of the additional segments will be moved at all.
Tape file migration can be thought of as a hybrid between the disk and tape volume migration
algorithms. Disk migration is a file based algorithm which is strictly concerned with making one
ormore copies of disk files. Tape volume migration is only concerned with freeing tape volumes by
movingdata segments from sparsely filled volumes either laterally or vertically.Tape file migration
is a file-based tape algorithm which is able to make a single copy of tape files to the immediately
lower level in the hierarchy. Similarly to disk migration, tape copies are made roughly in file
creation order, but the order is optimized to limit the number of tape mounts.
As with disk files, BFS creates migration records for tape files in storage classes which are using
tape file migration. MPS reads these migration records in the same manner as it does for disk.
Within a given storage class, files are always migrated in hierarchy and family order. Hierarchies
arecheckpointed in the same way as disk. Files are roughlymigrated by creation time (the time at
which the first file write completed), but priority is given to migrating all of the files off of the
current source volume over migrating files in time order.
Whena tape file migration run begins, all of the eligible migration records for the storage class are
read. For each migration record, the tape volume containing the corresponding file is identified. A
list of all of the source tape volumes which are to be migrated in the current run is created. MPS
then begins creating threads to perform the actual file migrations. A thread is created for each
source volume up to the limit specified byRequest Count in the migration policy. These threads
then read the migration records corresponding to their assigned volumes and migrate each file.
The migration threads end when the supply of migration records is exhausted. As each thread
ends,MPS starts another thread for the next source tape volume to be migrated. Themigration run
ends when all volumes in all hierarchies have been migrated.