6.Minimum Storage Segment Size Selection (Disk Only); 7.Maximum Storage Segment Size Selection (Disk Only) - IBM High Performance Storage System HPSS Installation Manual

streaming to maximize performance. For recommended values for various media types, see Section
3.10.1.12: Some Recommended Parameter Values for Supported Storage Media on page 106.
3.10.1.6. Minimum Storage Segment Size Selection (disk only)
The Core Server maps disk files onto a series of disk storage segments. The size of the storage
segments is controlled by the Min Storage Segment Size parameter, the Max Storage Segment Size
parameter, and the Average Number of Segments parameter. The smallest amount of disk storage
that can be allocated to a file is determined by the Min Storage Segment Size parameter. This
parameter should be chosen with disk space utilization in mind. For example, if writing a 4 KB file
into a Storage Class where the storage segment size is 1,024 KB, then 1,020 KB of the space will be
wasted. At the other extreme, each file can use at most 10,000 disk storage segments, so it isn't
possible to write a terabyte file to a disk Storage Class with a Maximum Storage Segment Size
below 128 megabytes. When file size information is available the Core Server will attempt to choose
an optimal storage segment size between Min Storage Segment Size and Max Storage Segment
Size with the goal of creating Average Number of Segments for the bitfile.
Guideline: Care should be taken when selecting the minimum storage segment size. If data will be
migrated from disks in the Storage Class to a tape Storage Class, the value of the Minimum Storage
Segment Size parameter should meet one of the following conditions. These rules help prevent the
creation of excessive numbers of tape storage segments when files are migrated from disk to tape.
If the storage segment size is larger than the tape stripe length, it should be an integer multiple
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of the tape stripe length. The storage segment size may be equal to the tape stripe length.
If the storage segment size is smaller than the tape stripe length, the tape stripe length should
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be an integer multiple of the storage segment size, and, it should be not more than 32 times the
storage segment size.
Guideline: When a large range of file sizes are to be stored on disk, define multiple disk storage
classes with appropriate storage segment sizes for the sizes of the files that are expected to be stored
in each Storage Class.
Explanation: The Class of Service (COS) mechanism can be used to place files in the appropriate
place. Note that although the Core Server provides the ability to use COS selection, current HPSS
interfaces only take advantage of this in two cases. First, the pput command in PFTP automatically
takes advantage of this by selecting a COS based on the size of the file. If the FTP implementation on
the client side supports the alloc command, a COS can also be selected based on file size. Files can
also be directed to a particular COS with FTP and PFTP commands by using the site setcos command
to select a COS before the files are stored. When setting up Classes of Service for disk hierarchies,
take into account both the Minimum Storage Segment Size parameter and the Maximum Storage
Segment Size parameter in determining what range of file sizes a particular COS will be configured
for.
3.10.1.7. Maximum Storage Segment Size Selection (disk only)
This parameter, along with Minimum Storage Segment Size and Average Number of Storage
Segments, is used by the Core Server to optimally choose a storage segment size for bitfiles on disk.
The largest storage segment size that can be selected for a file in a Storage Class is limited by this
parameter.
Guideline: In order to avoid creating excessive fragmentation of the space on disks in this Storage
Class, it is recommended that this be set no higher than 5% of the size of the smallest disk Virtual
Volume allocated in this Storage Class.
HPSS Installation Guide
Release 6.2 (Revision 2.0)
July 2008
105