Computer Drive User Manual

If a migration policy is added to a storage class after files are created in the storage class, those files
may never be migrated. Use the mkmprec utility to correct this problem. See the mkmprec man page for
more information.
Warning Threshold. A threshold for space used in this storage class expressed as a number of empty
tape volumes. Alarms will be sent to SSM periodically when the number of empty tapes in the storage
class falls below this value, and the Space Thresholds field on the HPSS Health and Status window will
be changed to Warning. Note that this field will not have any effect if overridden by Storage Subsystem-
Specific Thresholds.
Critical Threshold. Another threshold for space used in this storage class expressed as a number of
empty tape volumes. Alarms will be sent to SSM periodically when the number of empty tapes in the
storage class falls below this value, and the Space Thresholds field on the HPSS Health and Status
window will be changed to Critical. Note that this field will not have any effect if overridden by Storage
Subsystem-Specific Thresholds.
Optimum Access Size. The optimal transmission size to be used for a transfer request using this storage
class. (Not currently used by HPSS. May be used by site specific applications)
Average Latency. The average time (in seconds) that elapses when a data transfer request is scheduled
and the time the data transfer begins.
Maximum VVs to Write. The number of tape virtual volumes in the storage class that a Core Server
will use for concurrent writes.
Advice - Small values in this field restrict files being written in the storage class to a small number of
tapes, reducing the number of tape mounts. The number of tape drives used to write files in the storage
class will be limited to approximately the value of this field times the stripe width of the mounted VVs.
Read operations are not limited by this value.
Storage Segment Size
Media Type. The media type associated with the storage class.
Media Block Size (MBS). The Media Block Size is the size in bytes of a physical data block on the
media. For tape, this can be almost any value within reasonable limits. If the tape hardware has a
recommended physical block size, use that value.
Advice - The Media Block Size should be set to a value appropriate for the volume type. See the HPSS
Installation Guide, Section 3.10.1.1: Media Block Size Selection for some recommendations.
See also Device I/O Rate and Seconds Between Tape Marks in this section.
VV Block Size (VVBS). The virtual volume block size is the number of bytes written to an element of a
striped VV before the data stream moves to the next stripe element. It can be thought of as the stride
length of striped data transfer operations. The length of the VV block has an effect on the striping
efficiency. Short VV blocks cause more protocol overhead when writing striped devices. In non-striped
applications, VV Block Size has little meaning so any convenient multiple of the Media Block Size will
do.
Advice - When choosing a VV Block Size, the administrator should consider the characteristics of any
data source or sink the storage class that will be copied to or from. Best performance of striped copies
usually occurs when the VV Block Sizes of the source and sink are equal. This minimizes the data
HPSS Management Guide November 2009
Release 7.3 (Revision 1.0) 164