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VVs, fragmentation of the volumes may make it difficult to find space for a new segment. Setting
Average Number of Storage Segments to a larger value will increase the number of segments occupied
by files, and decrease the segment size. Fragmentation of the volumes will be reduced, but the amount of
metadata required to describe the files will be increased.
Conversely, if files are small relative to the size of the disk VVs, smaller values of Average Number of
Storage Segments increase the size of the storage segments, and decrease the number of segments
occupied by each file. This reduces the metadata storage requirements of the file.
The number of segments in small HPSS files can have a significant impact on transfer performance. To
maximize the transfer performance, set this value to 1. Keep in mind that this will result in less effective
disk space utilization. On average, you will use only 50% of your disk space with this selection.
Storage Segment Size
Media Type. The media type of all volumes in 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 disk, the value must be a multiple of the physical block size used by the disk hardware. For
example, if the disk hardware stores data in 512-byte sectors, 2048 would be a valid entry in this field,
but 2000 would not.
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.
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 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 movement protocol
overhead and helps to keep the data streams flowing smoothly.
VV Block Size must be an integer multiple of the Media Block Size.
See the HPSS Installation Guide, Section 3.10.1.2: Virtual Volume Block Size Selection (disk) and
Section 3.10.1.3: Virtual Volume Block Size Selection (tape) for information.
Stripe Width (SW). The number of Physical Volumes in a Virtual Volume in this Storage Class.
PV Size (PVSIZE). The size of the disk volume in bytes.
VV Size (VVSIZE). The virtual volume size. The product of the PV Size (PVSIZE) and the Stripe
Width (SW).
Stripe Length (SL). The Stripe Length is the product of the VV Block Size (VVBS) and the Stripe
Width (SW).
Min Multiplier (MINMULT). The Min Storage Segment Size (MINSEG) field is set to the product of
this value and Stripe Length (SL).
HPSS Management Guide November 2009
Release 7.3 (Revision 1.0) 161