Last Update Interval. The number of minutes that must pass since a file was last updated before it can
become a candidate for migration.
Number of Migration Streams Per File Family. The number of migration streams to be allocated to
each file family. This value effectively determines how many file families can be migrated
simultaneously. For example: Number of Migration Streams Per File Family is set to 2 and Total
Migration Streams is set to 10; up to five families will be migrated simultaneously (five families with
two streams working on each family yields a total of 10 migration streams).
Total Migration Streams. The number of parallel migration threads which will be run during migration
for each copy level in the hierarchy. This value must be a multiple of the Number of Migration Streams
Per File Family.
Advice - Setting the Total Migration Streams value too high may resul t in higher than desired tape drive
utilization. Each hierarchy within a disk storage class is migrated in a separate but parallel fashion. As
such, when there are files eligible for migration in each hierarchy withi n a disk storage class, tape
mount requests will be generated for each hierarchy. The Total Migration Str eams value may be used
indirectly to control how many tape mounts are requested, and thus how many drives are used during
migration. The maximum number of drives needed for a migration can be calculated as the stripe width
value (SW) multiplied by the Total Migration Streams value (TMS) for every hierarchy and disk-to-tape
copy level within the storage class:
( SW × TMS)Hier1,LevelA + ( SW × TMS )Hier1,LevelB + . . .
( SW × TMS)Hier2,LevelA + ( SW × TMS )Hier2,LevelB + . . .
+ . . . +
( SW × TMS )Hiern,Levelx + ( SW × TMS )Hiern,Levely + . . .
For example, an HPSS system that has a disk storage class which migrates to a tape st orage class with
stripe width two. The migration policy's request count is also two. In t his situation, the maximum tape
drive usage would be four: two sets of two-wide tape VVs.
A hierarchy that migrates data from disk to a first and second 4-wide tape copy with a request count of
one would use a maximum of eight tape drives: the first copy would go to one set of 4-wide tapes and
the second copy would migrate to a second set of 4-wide tapes. A second hierarchy added to the mix
with the same characteristics would double the number of tape drives needed.
Now consider an HPSS system that has disk Storage Class A which contains Hierarchy 1 and Hierarchy
2. Hierarchy 1 migrates to a tape storage class with stripe width of two and has a request count of four.
Hierarchy 2 migrates to a first copy tape storage class with stripe width of two and a request count of
three, and a second copy tape storage class with a stripe width of one and a request count of two. The
total drive utilization for a maximal migration case (when there exist files eligible for migration in all
hierarchies of the disk storage class) will be sixteen:
( 2SW × 4RC )Hier1,Level1 + ( 2SW × 3RC ) Hier2,Level1 + ( 1SW × 2RC )Hier2,Level2 = 16
In addition to the Request Count, the Maximum VVs to Write parameter in the Ta pe Storage Class
configuration window may be used to further control how many tape drives/cartridges are use for
migration: it will place a cap on the maximum number of VVs that can be written si multaneously in a
HPSS Management Guide November 2009
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