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SSM provides the capability to define storage classes and to add and delete virtual volumes to and
from the defined storage classes. A storage class is identified by a storage class ID and its associated
attributes. For detailed descriptions of each attribute associated with a storage class, see Section
6.1.1:

Configuring a Storage Class

of the

HPSS Management Guide

The sections that follow give guidelines and explanations for creating and managing storage classes.

3.10.1.1. Media Block Size Selection

Guideline

Select a block size that is smaller than or equal to the maximum physical block size that a

device driver can handle.

Explanation

: For example, see Section 3.10.1.12:

Some Recommended Parameter Values for

Supported Storage Media

on page 106 for recommended values for tape media supported by HPSS.

3.10.1.2. Virtual Volume Block Size Selection (disk)

Guideline

The virtual volume block size must be a multiple of the underlying media block size.

Explanation

: This is needed for correct operation of striped I/O. It may be necessary to experiment

with combinations of disk and tape VV Block Sizes to find combinations that provide maximum
transfer performance

3.10.1.3. 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 an I/O transfer buffer size such that the size of the buffer being used to write this

Storage Class is an integer multiple of the block size.

Explanation

: 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 operation. 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 Core Server will create a separate storage segment for the data associated with each write, which
wastes metadata space. Performing these extra steps will degrade transfer performance. See also
Section 3.10.1.12:

Some Recommended Parameter Values for Supported Storage Media

on page 106

for further information about selecting block sizes.

Guideline 3

: Disk and tape VV block sizes should be equal if possible.

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.

3.10.1.4. 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

For tapes, the stripe width should be less than half the number of available tape drives.

Explanation

: There must be enough tape drives to support the Stripe Width. The

repack

and

recover

HPSS Installation Guide

July 2008

Release 6.2 (Revision 2.0)