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Chapter 2 HPSS Planning
HPSS Installation Guide September 2002 97
Release 4.5, Revision 2
Explanation: The Class of Service (COS) mechanism can be used to place files in the appropriate
place. Note that although the Bitfile 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 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.
NFS is more of a challenge in this area. NFS puts all the files it creates into a particular class of
service, independent of the file size. If a fileset has a class of service associated with it, files will be
put into that class of service. Otherwise, the daemon’s class of service will be used. Sites may want
to set up different filesets to deal with large and small files, and assign a class of service accordingly.
It is important to use the Storage Segment Size, Maximum Storage Segment Size, and Average
Number of Segments parameters to allow for the range of file sizes that clients typically store in the
fileset. Remember that NFS V2 files cannot be larger than 2GB.
2.9.1.7 Maximum Storage Size Selection (disk only)
This parameter, along with Storage Segment Size and Average Number of Storage Segments, is
used by the Bitfile 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 parameter be set no higher that 5% of the size of the smallest disk
allocated to this storage class.
2.9.1.8 Maximum VVs to Write (tape only)
This parameter restricts the number of tape VVs, per storage class, that can be concurrently written
by the Tape Storage Server. The purpose of the parameter is to limit the number of tape VVs being
written to prevent files from being scattered over a number of tapes and to minimize 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 defined for the storage class. Note that this field only
affects tape write operations. Read operations are not limited by the value defined by this
parameter.
2.9.1.9 Average Number of Storage Segments (disk only)
This parameter, along with Storage Segment Size and Maximum Storage Segment Size, is used by
the Bitfile Server to optimally choose a storage segment size for bitfiles on disk. The Bitfile Server
attempts to choose a storage segment size between Storage Segment Size and Maximum Storage
Segment Size that would result in creating the number of segments indicated by this parameter.
Guideline: For best results, it is recommended that small values (< 10) be used. This results in
minimizing metadata created and optimizing migration performance. The default of 4 will be
appropriate in most situations.