IBM Hub/Switch Installation Manual page 94

Chapter 2 HPSS Planning
Explanation: For example, if a site has ESCON attached tape drives on an RS6000, the driver can
handle somewhat less than 64 KB physical blocks on the tape. A good selection here would be
32 KB. See Section 2.9.1.12 for recommended values for tape media supported by HPSS.
2.9.1.2 Virtual Volume Block Size Selection (disk)
Guideline: The virtual volume (VV) block size must be a multiple of the underlying media block
size.
Explanation: This is needed for correct operation of striped I/O.
2.9.1.3 Virtual Volume Block Size Selection (tape)
Guideline 1: The VV block size must be a multiple of the media block size
Explanation: This is needed for correct operation of striped I/O.
Guideline 2: Pick a block size such that the size of the buffer that is being used by writers to this
storage class is an integral multiple of the block size.
Explanation: For example, assume files are being written via standard FTP directly into a tape
storage class. Also assume FTP is set up to use a 4 MB buffer size to write the data. This means that
writes are done to the tape with a single 4 MB chunk being written on each write call. If the tape
virtual volume block size is not picked as indicated by the guideline, two undesirable things will
happen. A short block will be written on tape for each one of these writes, which will waste data
storage space, and the Storage Server will build a separate storage segment for the data associated
with each write, which will waste metadata space. See also Section 2.8.1.6 for further information
about selecting block sizes.
Guideline 3: Disk and tape VV block sizes should be chosen so when data is migrated from disk to
tape, or tape to disk, the VV block size doesn't change.
Explanation: The system is designed to maximize throughput of data when it is migrated from disk
to tape or tape to disk. For best results, the sizes of the VV blocks on disk and tape in a migration
path should be the same. If they are different, the data will still be migrated, but the Movers will be
forced to reorganize the data into different size VV blocks which can significantly impact
performance.
2.9.1.4 Stripe Width Selection
Stripe width determines how many physical volumes will be accessed in parallel when doing read/
writes to a storage class.
Guideline 1: On tape, the stripe width should be less than half the available drives if multiple sets
of drives are required for certain operations.
Explanation: There must be enough tape drives to support the stripe width selected. If planning to
run tape repack on media in this storage class, the stripe width cannot be greater than half the
number of drives available. In addition, if doing tape-to-tape migration between two storage
classes that have the same media type and thus potentially share the same drives, the stripe width
94 September 2002 HPSS Installation Guide
Release 4.5, Revision 2